Taken from aerodynamics for naval aviators: "The equivalent airspeed (EAS) is the flight speed in the standard sea level air mass which would produce the same free stream dynamic pressure as the actual flight condition".
The standard airspeed indicator is calibrated to account for compressibility effects at sea level in the standard atmosphere. However when ambient conditions are different from standard sea level additional corrections have to be made to calibrated airspeed in order to determine the actual dynamic pressure acting on the aircraft.