Plain English Meaning

The single airspeed where the wing is working most efficiently — getting the most lift for the least drag. If the engine quits, this is the speed that lets the airplane glide the farthest.

Full Definition

The specific angle of attack and airspeed at which an airplane produces the greatest amount of lift for the least amount of drag. At this point, the ratio of lift to drag is at its highest value, which corresponds to the most efficient flight condition aerodynamically. In a power-off glide, flying at L/DMAX produces the maximum glide distance for a given altitude.

Context Anchor

Seen in glide performance discussions, especially when learning and how the airplane can glide after a loss.