noun
sidelight; side light; running light
Nautical term for the red (port) and green (starboard) navigation lights on a vessel.
夜間航行中は舷灯を点灯しなければならない。
Sidelights must be lit during nighttime navigation.
左舷の舷灯は赤、右舷は緑です。
The port sidelight is red, and the starboard one is green.
航海灯 is a broader term for all navigation lights, including masthead and stern lights, while 舷灯 specifically refers to the side lights.
舷 (げん, 'gunwale, side of a ship') + 灯 (とう, 'light'). A straightforward compound describing the side-mounted navigation lights on a vessel.