Drag

The aerodynamic force that opposes an aircraft's motion through the air, acting rearward and parallel to the relative wind. There are four types: parasite drag (caused by the aircraft's shape and surface moving through the air, which increases with speed), induced drag (a byproduct of lift production that decreases with speed), form drag (a subset of parasite drag caused by the shape of objects disrupting airflow), and skin friction drag (caused by air molecules rubbing against the aircraft surface). Total drag is lowest at the speed where parasite and induced drag are equal, which is the best lift-over-drag (L/Dmax) speed.