slipstream

A slipstream is a region behind a moving object in which a wake of fluid (typically air or water) is moving at velocities comparable to the moving object, relative to the ambient fluid through which the object is moving. The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "drafting" works because of the relative motion of the fluid in the slipstream.