The International Regulations for Preventing Collisions at Sea (IRPCS or COLREGs) are mandatory international rules designed to prevent collisions between vessels, covering navigation, lights, sound signals, and right-of-way hierarchies. They apply to all vessels, from small pleasure craft to large commercial ships.

Key IRPCS Definitions & Terms:
Vessel: Every description of watercraft, including non-displacement craft and seaplanes, capable of being used as transportation on water.

Power-driven vessel: Any vessel propelled by machinery.

Sailing vessel: Any vessel under sail provided that propelling machinery, if fitted, is not being used.

Vessel engaged in fishing: Any vessel fishing with nets, lines, trawls, or other apparatus that restrict maneuverability.

Vessel not under command (NUC): A vessel that, through exceptional circumstances, is unable to maneuver as required by the rules.

Vessel restricted in her ability to manoeuvre (RAM): A vessel restricted by the nature of her work (e.g., dredging, surveying, cable laying).

Underway: A vessel that is not at anchor, made fast to the shore, or aground.

In sight of one another: Vessels are only deemed to be in sight of one another when one can be observed visually from the other.