Pardon my ignorance, but I can't remember what "EAS" is (not sure if I ever learned this term in my PPL training actually)...
EAS = Equivalent Airspeed. If you read the standard private pilot ground school stuff you'll get nice (but useless) words on the difference between TAS and EAS, and EAS and CAS, like "EAS is CAS corrected for compressibility" - that sure clears things up
Lets look at this from an aerodynamics point of view. If we look at the standard equations for lift and drag, we find that they start with 1/2 times the density times the speed (1/2 rho V^2, where rho is how you pronounce that Greek symbol for density). This term, 1/2 rho V^2, is the dynamic pressure - this is the pressure over and above the ambient pressure that is created when the moving air "hits" the aircraft. It is a very important parametre that determines lift, drag, aerodynamic forces, etc. If the air that we flew in was an incompressible fluid, the dynamic pressure would depend on CAS. In this case, our stall speeds, lift, drag, aerodynamic loadings on surfaces, etc would all depend exactly on CAS. But, the wonderful air that lifts us is compressible. In the world of compressible fluids, the dynamic pressure varies exactly with EAS instead of CAS. The pilot ground school stuff hint at this when they talk about a compressibility correction to get from CAS to EAS.
If EAS is so important, why do our airspeed indicators show CAS, and not EAS? That is because there is no simple mechanical design that can take pitot and static pressure and output EAS. A CAS indicator just needs a diaphragm with pitot pressure on one side, and static pressure on the other side, and a gearing mechanism to convert movements of the diaphragm to movement of a needle.
The difference between CAS and EAS is not practically important to us RV folks, as EAS is almost exactly the same as CAS in the airspeed and altitude range that RVs operate at. For example, at 200 kt CAS at 10000 ft, the difference between CAS and EAS is one kt. The difference between CAS and EAS becomes greater at higher speeds and altitudes. At typical airliner speeds and altitudes the difference between CAS and EAS is about 10 kt. At very high speeds and altitudes, the difference becomes quite important, which is why SR-71s have airspeed indicators that read in EAS rather than CAS.
I've cut a few corners here - the full story, with all relevant tangential discussions, would take many pages.