Maritime Glossary

Definitions combine cited rule passages with study summaries. Read the source, its scope and exceptions; study translations are labelled and are not official instruments.

49 terms

Vessel types & status

VesselCOLREG Rule 3(a): (a) The word vessel includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Power-driven vesselCOLREG Rule 3(b): (b) The term power-driven vessel means any vessel propelled by machinery.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Sailing vesselCOLREG Rule 3(c): (c) The term sailing vessel means any vessel under sail provided that propelling machinery, if fitted, is not being used.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Vessel engaged in fishingCOLREG Rule 3(d): (d) The term vessel engaged in fishing means any vessel fishing with nets, lines, trawls or other fishing apparatus which restrict maneuverability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restrict maneuverability.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Not under command (NUC)COLREG Rule 3(f): (f) The term vessel not under command means a vessel which through some exceptional circumstance is unable to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Restricted in ability to manoeuvre (RAM)COLREG Rule 3(g): (g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Constrained by her draught (CBD)COLREG 3(h), 28: (h) The term vessel constrained by her draft means a power-driven vessel which, because of her draft in relation to the available depth and width of navigable water is severely restricted in her ability to deviate from the course she is following. A vessel constrained by her draft may, in addition to the lights prescribed for power-driven vessels in Rule 23, exhibit where they can best be seen three all-round red lights in a vertical line, or a cylinder.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
UnderwayCOLREG Rule 3(i): (i) The word underway means that a vessel is not at anchor, or made fast to the shore, or aground.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Making way through the water: Motion relative to the surrounding water, distinct from being underway. Drifting does not by itself establish whether a vessel is making way: consider its motion through the water, not only its speed over the ground or whether its engine is running.Study summary · COLREG 3(i), 27, 35(a)–(b)
WIG craftCOLREG Rule 3(m): (m) The term Wing-In-Ground (WIG) craft means a multimodal craft which, in its main operational mode, flies in close proximity to the surface by utilizing surface-effect action.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
COLREGSIMO 1972: The 1972 Convention on the International Regulations for Preventing Collisions at Sea and its rules, as amended. (a) These Rules shall apply to all vessels upon the high seas and in all waters connected therewith navigable by seagoing vessels. (b) Nothing in these Rules shall interfere with the operation of special rules made by an appropriate authority for roadsteads, harbors, rivers, lakes or inland waterways connected with the high seas and navigable by seagoing vessels. Such special rules shall conform as closely as possible to these Rules.Study summary · U.S. Coast Guard — International Navigation Rules (2024)

Steering & sailing

Give-way vesselCOLREG Rule 16: Every vessel which is directed to keep out of the way of another vessel shall, so far as possible, take early and substantial action to keep well clear.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Stand-on vesselCOLREG Rule 17: (a) (i) Where one of two vessels is to keep out of the way the other shall keep her course and speed. (ii) The latter vessel may, however, take action to avoid collision by her maneuver alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules. (b) When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Head-on situationCOLREG Rule 14: (a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision each shall alter her course to starboard so that each shall pass on the port side of the other. Rules in this section apply to vessels in sight of one another.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Crossing situationCOLREG Rule 15: When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel. Rules in this section apply to vessels in sight of one another.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
OvertakingCOLREG Rule 13: (b) A vessel shall be deemed to be overtaking when coming up with another vessel from a direction more than 22.5° abaft her beam, that is, in such a position with reference to the vessel she is overtaking, that at night she would be able to see only the sternlight of that vessel but neither of her sidelights. (d) Any subsequent alteration of the bearing between the two vessels shall not make the overtaking vessel a crossing vessel within the meaning of these Rules or relieve her of the duty of keeping clear of the overtaken vessel until she is finally past and clear.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Risk of collisionCOLREG Rule 7: (a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist. (d) In determining if risk of collision exists the following considerations shall be among those taken into account: (i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change; (ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Safe speedCOLREG Rule 6: Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Close-quarters situationCOLREG Rule 8(d): An encounter in which vessels become dangerously close in the prevailing circumstances. COLREG gives no universal distance threshold; assess the situation early and take effective action to pass at a safe distance.Study summary · COLREG 8(d), 19(d)–(e)
Action in extremisCOLREG Rule 17(b): (b) When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
In sight of one anotherCOLREG Rule 11: (k) Vessels shall be deemed to be in sight of one another only when one can be observed visually from the other.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Restricted visibilityCOLREG Rule 19: (l) The term restricted visibility means any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes. (a) This Rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Narrow channelCOLREG Rule 9: (a) A vessel proceeding along the course of a narrow channel or fairway shall keep as near to the outer limit of the channel or fairway which lies on her starboard side as is safe and practicable. (b) A vessel of less than 20 meters in length or a sailing vessel shall not impede the passage of a vessel which can safely navigate only within a narrow channel or fairway. (c) A vessel engaged in fishing shall not impede the passage of any other vessel navigating within a narrow channel or fairway.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Traffic Separation Scheme (TSS)COLREG Rule 10: (a) This Rule applies to traffic separation schemes adopted by the Organization and does not relieve any vessel of her obligation under any other rule. (b) A vessel using a traffic separation scheme shall: (i) proceed in the appropriate traffic lane in the general direction of traffic flow for that lane; (ii) so far as practicable keep clear of a traffic separation line or separation zone; (iii) normally join or leave a traffic lane at the termination of the lane, but when joining or leaving from either side shall do so at as small an angle to the general direction of traffic flow as practicable.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Demarcation line33 CFR Part 80: In the United States, COLREG Demarcation Lines separate waters governed by the International Rules from those governed by US Inland Rules. Their legally defined locations are in 33 CFR Part 80; consult the applicable chart and rules.Study summary · 33 CFR Part 80

Lights & shapes

Masthead lightCOLREG Rule 21(a): (a) Masthead light means a white light placed over the fore and aft centerline of the vessel showing an unbroken light over an arc of the horizon of 225° and so fixed as to show the light from right ahead to 22.5° abaft the beam on either side of the vessel.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
SidelightsCOLREG Rule 21(b): (b) Sidelights means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5° and so fixed as to show the light from right ahead to 22.5° abaft the beam on its respective side. In a vessel of less than 20 m in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
SternlightCOLREG Rule 21(c): (c) Sternlight means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135° and so fixed as to show the light 67.5° from right aft on each side of the vessel.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Towing lightCOLREG Rule 21(d): (d) Towing light means a yellow light having the same characteristics as the sternlight defined in paragraph (c) of this Rule.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
All-round lightCOLREG Rule 21(e): (e) All-round light means a light showing an unbroken light over an arc of the horizon of 360°.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Flashing lightCOLREG Rule 21(f): (f) Flashing light means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)

Sound & light signals

Short blastCOLREG Rule 32(b): (b) The term short blast means a blast of about one second’s duration.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Prolonged blastCOLREG Rule 32(c): (c) The term prolonged blast means a blast of from four to six seconds’ duration.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
Danger / doubt signalCOLREG Rule 34(d): (d) When vessels in sight of one another are approaching each other and from any cause either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. Such signal may be supplemented by a light signal of at least five short and rapid flashes.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)
WhistleCOLREG Rule 32(a): (a) The word whistle means any sound signaling appliance capable of producing the prescribed blasts and which complies with the specifications in Annex III to these Regulations.Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024)

IALA buoyage

Lateral markIALA MBS: The lateral colours change: in Region A, port-hand marks are red and starboard-hand marks green; in Region B, port is green and starboard red, following the conventional direction of buoyage. The other mark families use the same system in both regions.Study summary · IALA R1001, ed. 2.0 (2023)
Cardinal markIALA MBS: Cardinal marks indicate the direction of safe water relative to the mark using compass quadrants (N, E, S, W).Study summary · IALA R1001, ed. 2.0 (2023)
Isolated danger markIALA MBS: An isolated danger mark is on or near a danger with navigable water around it. Consult charts and nautical publications for the extent of the danger and safe passing distance. Navigable water exists all around the markStudy summary · IALA R1001, ed. 2.0 (2023)
Safe water markIALA MBS: Safe water mark: red and white vertical stripes with a red sphere, navigable water all around.Study summary · IALA R1001, ed. 2.0 (2023)
Special markIALA MBS: Yellow marks indicating special areas such as ODAS, traffic separation, military exercise areas, or cables/pipelines. A special mark may also identify a dangerous floating object as a MAtoNStudy summary · IALA R1001, ed. 2.0 (2023)
Emergency wreck marking buoyIALA MBS: The emergency wreck marking buoy marks new dangers, including newly discovered wrecks. The competent authority determines further marking once the danger is removed or its information has been sufficiently promulgated. 1 s blue + 0.5 s darkness + 1 s yellow + 0.5 s darkness; period 3 s.Study summary · IALA R1001, ed. 2.0 (2023)
IALA Region A & Region BIALA MBS: The lateral colours change: in Region A, port-hand marks are red and starboard-hand marks green; in Region B, port is green and starboard red, following the conventional direction of buoyage. The other mark families use the same system in both regions.Study summary · IALA R1001, ed. 2.0 (2023)

GMDSS & radio

GMDSSSOLAS Ch. IV: The Global Maritime Distress and Safety System combines terrestrial and satellite radio services, ship equipment and procedures for distress and safety communications. SOLAS Chapter IV defines its functions and carriage requirements; it is more than an automatic ship-to-shore alarm.Study summary · SOLAS IV/4
Digital Selective Calling (DSC): A digital calling and alerting technique on VHF, MF and HF, distinct from subsequent voice or data traffic. Depending on the call category it can address an individual station, a group or all ships; a distress alert is not restricted to one recipient MMSI.Study summary · ITU-R M.493
MMSI: A nine-digit identity used in maritime radio services. Ship stations use MIDXXXXXX, coast stations 00MIDXXXX and SAR aircraft 111MIDXXX: the three-digit MID identifies the responsible administration, but is not always the first three digits of the identity.Study summary · ITU-R M.585
EPIRB: A 406 MHz maritime distress beacon that alerts Cospas-Sarsat; 121.5 MHz supports nearby homing. GNSS, AIS locating capability and automatic release/activation depend on the model and applicable standard: the modern IMO standard for installations from 1 July 2022 includes GNSS and AIS; follow the equipment instructions.Study summary · IMO MSC.471(101); MSC.514(105)
SART: A search and rescue locating device. A radar SART responds to X-band radar with twelve marks extending away from its approximate position: the innermost mark, nearest the radar, indicates that position; AIS-SART instead broadcasts locating information on AIS frequencies.Study summary · IMO MSC.510(105); MSC.246(83)
NAVTEX: A broadcast service for automatic reception of maritime safety information. International NAVTEX uses 518 kHz in English; national services may use 490 or 4209.5 kHz, including national-language broadcasts.Study summary · USCG NAVTEX
STCWIMO 1978: The International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (1978, as amended) sets minimum international standards for seafarers. STCW Code Part A is mandatory; Chapter VIII covers watchkeeping and Chapter VI includes emergency, safety, security, medical-care and survival functions, including basic training.Study summary · IMO STCW Convention and Code

How this glossary is built

Definitions combine cited rule passages with study summaries. Read the source, its scope and exceptions; study translations are labelled and are not official instruments.

Each term has a stable anchor. For example, /en/glossary#not-under-command links directly to the NUC definition; use the corresponding locale prefix for another language.

Glossary FAQ

Why does the same term sometimes mean different things in COLREG and IALA?

Terms must be read in their source context. COLREG regulates collision avoidance, IALA buoyage identifies navigational marks, and GMDSS concerns maritime distress and safety communications; similar words do not make their definitions interchangeable.

Are these definitions accepted for exam answers?

These are study definitions, not a guarantee of accepted examination wording. Check the current syllabus and required wording of your examination authority, including national rules and the status of any translation.

Can I link to a single term?

Each term has a stable anchor. For example, /en/glossary#not-under-command links directly to the NUC definition; use the corresponding locale prefix for another language.