Equivalent airspeed

The calibrated airspeed of an aircraft corrected for adiabatic compressible flow for the particular altitude. Equivalent airspeed is equal to calibrated airspeed in standard atmosphere at sea level. (FAR definition).

Airspeed equivalent to CAS in standard atmosphere at sea level. As the airspeed and pressure altitude increase, the CAS becomes higher than it should be, and a correction for compression must be subtracted from the CAS. (PHAK definition).

Source: 14 CFR Federal Aviation Regulations §1, Pilot Handbook of Aeronatical Knowledge