Definition
The airspeed at which total drag on the airplane is at its lowest value. At this speed, induced drag (the drag produced by generating lift) and parasite drag (the drag from the airplane pushing through the air) are equal, and their sum is smaller than at any other speed. Flying slower than VMD causes induced drag to rise sharply; flying faster causes parasite drag to rise. VMD also defines the boundary between the normal speed range (faster than VMD) and the region of reversed command, sometimes called the back side of the power curve (slower than VMD).
Plain English
The speed at which the airplane meets the least resistance from the air. Fly slower or faster than this speed and the airplane has to work harder to keep going.
Context Anchor
Seen in low-speed flight and performance discussions, especially when explaining why an airplane may need more power as it slows down.
Derivation
Drag is the rearward force resisting the airplane's motion through the air. 'Minimum drag speed' is literally the speed where that force is smallest. The 'V' in VMD follows the standard aviation convention of using V (from the Latin velocitas, meaning speed) to label specific reference airspeeds.
Why Pilots Care
It produces the maximum glide distance and is the reference for efficient flight in the low-speed regime.
Grounding Statement
Picture slowing an airplane in level flight: at first less speed may seem easier, but below VMD the airplane has to work harder to keep flying level.
Intuition Check
Minimum drag speed does not mean the slowest safe speed. It means the speed where total air resistance is lowest; slower than VMD can actually create more drag.
Example Sentence 1
During slow flight practice, the instructor pointed out that the airplane was flying below VMD, which is why more power was needed to hold altitude as airspeed decreased.
Example Sentence 2
During a power-off approach the pilot adjusts pitch to hold VMD until the flare.