Equivalent airspeed (EAS) is defined as the speed at sea level, under ISA conditions, that would produce the same incompressible dynamic pressure that is produced at the true airspeed and the altitude at which the vehicle is flying.
Equivalent airspeed can be defined in terms of true airspeed, as above, but it is also defined in terms of calibrated airspeed (CAS) corrected for adiabatic compressible flow at the altitude of flight. At sea level under ISA conditions, equivalent airspeed and calibrated airspeed are the same. At any other altitude, equivalent airspeed will be less than calibrated airspeed. Below speeds of about 200 knots CAS and at altitudes below 10,000', the difference between CAS and EAS is negligible. However, at higher speeds and altitudes CAS must be corrected for compressibility error to determine EAS.
The significance of equivalent airspeed is that, at Mach numbers below the onset of wave drag, all of the aerodynamic forces and moments on an aircraft are proportional to the square of the equivalent airspeed.