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; as, the Ambrose lightship off New York was rammed and damaged in 1950 by the Santa Monica.

A vessel used to mark a hazardous area and/or guide shipping traffic, anchored in position with a mast mounted beacon and equipped with a steam whistle for fog warning.

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