Warp current

A warp current, or subspace current, is an area of where subspace is in a state of motion relative to normal space. The currents affect the relative speed of starships traveling within them.

A ship traveling with the flow of a current is given an extra 'push' with additional energy added to the warp field. Likewise, a ship traveling into a current requires a stronger warp field to overcome the current's effects.

In 2377, the USS Baldwin (NCC-2013) encountered a 25 kilocochrane current within Cardassian space. The current swept Baldwin into a subspace sandbar in the Beta Quadrant, where they found the starship Discovery. In order to escape this sandbar, a starship would have to maintain warp 9.996 to overcome the currents effects, something which Discovery was incapable of doing. (: "Against the Wind")

Background
The concept of warp currents is analogous to traditional ocean currents, or to the headwinds and tailwinds encountered by aircraft.