Plain English Meaning

Equivalent airspeed is the airspeed reading after a small correction is applied for the way air gets squeezed against a fast-moving aircraft at high altitude. It tells you the airspeed value that matches the actual aerodynamic forces acting on the wings.

Full Definition

Equivalent airspeed is calibrated airspeed corrected for the compressibility of air at the aircraft's current altitude and speed. At low speeds and low altitudes, equivalent airspeed and calibrated airspeed are essentially the same. As speed and altitude increase, air begins to compress against the aircraft, causing the airspeed indicator to read slightly higher than the true would suggest. Removing that gives equivalent airspeed.