Definition
On a multi-engine aircraft, the engine whose failure would most adversely affect the performance and handling qualities of the aircraft.
Plain English
On a plane with more than one engine, it's the engine that — if it quits — causes the worst handling and performance problems. Losing this particular engine is harder to deal with than losing any of the others.
Context Anchor
Seen in multi-engine training, aircraft manuals, and emergency procedures for engine failure after takeoff, during climb, or in other high-workload situations.
Derivation
Critical' here comes from the Latin 'criticus', meaning decisive or crucial — a turning point. In aviation, it flags the engine whose loss creates the decisive handling problem, not just any engine failure.
Why Pilots Care
Failure of this engine requires the highest minimum control speed and demands immediate, precise corrective action to maintain directional control.
Intuition Check
Critical does not mean the engine is damaged, weak, or more likely to fail. It means that if that engine does fail, its loss has the worst effect on the aircraft.
Example Sentence 1
On most conventional twins, the left engine is the critical engine because its failure causes a larger yawing moment than failure of the right.
Example Sentence 2
The pilot confirmed the right engine was not critical before practicing the engine-out drill.