A lightship is a small ship that stays in one place and has a powerful flashing lamp. Lightships are used to guide ships or to warn them of danger.
lightship in British English
(ˈlaɪtˌʃɪp)
noun
a ship equipped as a lighthouse and moored where a fixed structure would prove impracticable
lightship in American English
(ˈlaɪtˌʃɪp)
noun
a ship moored in a place dangerous or important to navigation and bearing a light or lights and foghorns, sirens, etc. by which ships are warned or guided