The area behind a vehicle moving through something where there is not so much air resistance (if moving through air. This can be used to gain speed before overtaking someone in a race.
Also if in space when your spaceship is in the slipstream of another it can't be detected by the other ship's sensors (but this is only if you are in an episode of star trek).
While having sex with a women doggystyle, grab her hair so she can't move her head then signal a friend to slide in as you pull out, he then finishes your job.