Overview
Drafting or slipstreaming is an aerodynamic technique where two moving objects are aligning in a close group to exploit the lead object's slipstream and thus reduce the overall effect of drag. Especially when high speeds are involved, as in motor racing and cycling, drafting can significantly reduce the paceline's average energy expenditure and can even slightly reduce the energy expenditure of the lead vehicle.
Drafting in sport
Drafting is used to reduce wind resistance and is seen most commonly in bicycle racing, motorcycle racing, car racing, and speedskating, though drafting is occasionally used even in cross-country skiing, downhill skateboarding, and running. Some forms of triathlon allow drafting. Drafting occurs in swimming as well: both in open-water races (occurring in natural bodies of water) and in traditional races in competition pools. In a competition pool a swimmer may hug the lane line that separates them from the swimmer they are abaft of thereby taking advantage of the liquid slipstream in the other swimmer's wake. Drafting also occurs in competitive longboarding.
It is believed, but not yet conclusively proven, that thoroughbred racing horses draft each other, especially in longer races.
Cycling
From Wikipedia (CC BY-SA 4.0).