Lightship
/ˈlaɪtʃɪp/
Definition 1 of 1
Pro Tip 1/3Maritime Navigation
Lightships are used for maritime navigation; think of them as movable lighthouses in the sea. 
The old sailor told stories of how the lightship's glow led them home through fog.
Pro Tip 2/3Danger Zones
Lightships mark dangerous areas, like shallow waters where ships could run aground. 
He checked the map for the lightship's position to steer clear of the rocky seabed.
Pro Tip 3/3Fixed Position
Unlike normal ships, lightships stay anchored in one location to serve as a consistent marker. 
The captain noted the lightship hadn't moved, a constant in the changing sea.