lightship

A ship with a powerful light or warning signals that is anchored in dangerous waters to alert other vessels.

A vessel riding at anchor and displaying a light for the guidance of mariners, in a position where the bottom or the depth would render a fixed lighthouse-structure impracticable.

A vessel equipped like a lighthouse, carrying at the masthead a brilliant light, and moored off a shoal or place of dangerous navigation where a permanent lighthouse would be impracticable, to serve as a guide for mariners.

A ship equipped with a very large lamp; it can be positioned to warn off other ships from dangerous locations, as a sort of portable lighthouse.

a ship equipped like a lighthouse and anchored where a permanent lighthouse would be impracticable