Plain English Meaning
A blue mark on the airspeed indicator of a twin-engine airplane that shows the speed to fly if one engine quits. At that speed the airplane climbs best on the remaining engine.
Full Definition
On a multi-engine airplane airspeed indicator, the blue radial line marking VYSE — the best rate-of-climb airspeed with one engine inoperative. Flying at or above this speed gives the airplane its best chance of climbing, or at least minimizing altitude loss, after losing an engine.
Context Anchor
Seen on the airspeed indicator and in multiengine training, especially during engine-failure practice and performance discussions.
Derivation
The term comes from the actual blue line or blue mark printed on the airspeed indicator. Pilots use the as a quick for the one-engine best-.