lightship /(līt"shĭp`)/

light·ship

lightship

n.
  1. 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. (Naut.)