Definition
A departure from an assigned or target altitude, where the aircraft is higher or lower than the altitude the pilot is required to maintain. In instrument flying, even small deviations are significant because ATC clearances and IFR procedures depend on precise altitude keeping.
Plain English
The airplane is not at the altitude it is supposed to be at. It has drifted above or below the height the pilot was told to hold.
Context Anchor
Seen during instrument flying on an electronic flight display when the pilot uses pitch control to hold an altitude or return to the correct altitude.
Derivation
From Latin deviare, meaning 'to turn off the road' (de-, away from, plus via, road). An altitude deviation is the aircraft straying from the altitude path it was assigned.
Why Pilots Care
Uncorrected altitude deviations can cause airspace violations, traffic conflicts, or an unsatisfactory approach or checkride result.
Intuition Check
Do not confuse altitude deviation with pitch attitude. Pitch attitude is where the nose is pointed; altitude deviation is the result: how far the airplane is above or below the altitude it should be at.
Example Sentence 1
The instructor pointed out a 200-foot altitude deviation on the climb-out and reminded the student to cross-check the altimeter more often.
Example Sentence 2
During the precision approach, the examiner noted an altitude deviation of more than 100 feet and asked for an immediate correction.