Definition
Calibrated airspeed corrected for the compressibility of air at higher speeds and altitudes. Equivalent airspeed (EAS) equals true airspeed at standard sea level conditions, and is used in performance and structural calculations where the dynamic pressure on the airframe matters more than the indicated reading.
Plain English
It is the airspeed reading after it has been adjusted for the fact that air gets squeezed (compressed) when an aircraft moves through it quickly. This adjustment makes the speed value reflect the actual force the air is exerting on the airframe.
Context Anchor
Seen in performance, high-altitude flight, high-speed flight, and structural speed-limit discussions.
Derivation
From Latin aequus (equal) and valere (to be worth) -- 'of equal value.' The name reflects what the speed is equivalent to: the airspeed that would produce the same dynamic pressure on the airframe at standard sea level conditions.
Why Pilots Care
It keeps stall speeds, maneuvering limits, and structural loads consistent regardless of altitude or density.
Grounding Statement
At a high altitude, an airplane may be moving very fast through thin air, while its equivalent airspeed is lower because the thin air produces less pressure on the airplane.
Intuition Check
Equivalent does not mean “roughly the same” here. It means a converted airspeed that gives the same air-pressure effect on the airplane under standard sea-level conditions.
Example Sentence 1
At high altitude, the pilot noted that true airspeed was much greater than equivalent airspeed because the thinner air produced less dynamic pressure on the airframe.
Example Sentence 2
Aircraft manuals list maximum operating speeds in equivalent airspeed to maintain safe structural margins.