Definition
In a multi-engine airplane, the airspeed that delivers the greatest gain in altitude per unit of time when one engine is inoperative and the remaining engine is producing maximum available thrust. It is marked on the airspeed indicator with a blue radial line, commonly called the 'blue line.'
Plain English
If one engine quits in a twin, this is the speed that gets you climbing — or losing altitude the slowest — as efficiently as possible on the engine that is still working.
Context Anchor
You see VYSE in multiengine airplane performance data, emergency procedures, and on many airspeed indicators as the blue-line speed.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed, used internationally for airspeed references. 'Y' is the standard FAA designator for best rate of climb (as opposed to 'X' for best angle of climb). 'SE' simply marks it as the single-engine version. So VYSE reads as 'speed for best rate of climb, single engine.'
Why Pilots Care
Maintaining this speed gives the greatest climb performance during an engine failure, helping clear obstacles and reach a safe altitude.
Intuition Check
Do not read “single-engine” here as meaning a normal one-engine airplane. In this context, it means a multiengine airplane flying with one engine failed or not producing power.
Example Sentence 1
After the right engine failed, the pilot pitched for the blue line and held VYSE while securing the failed engine.
Example Sentence 2
During the engine-out drill the instructor had the student hold single-engine best rate-of-climb until reaching pattern altitude.