fer·ry
-noun
a commercial service with terminals and boats for transporting persons, automobiles, etc., across a river or other comparatively small body of water.

A ferry in terms of mass transit is a boat that transports people across a channel, bay, river, etc. where possible. Ferries in Chicago for example transport passengers from the lake front to various sites along the Chicago River. Ferries are an effective way of moving large quantities of passengers across a body of water and along a route that would not be possible using any other type of transportation.

Ferries have been in operation since the early 19th century and continue to play an important role in a city's mass transit options when a body of water is between destinations.