Plain English Meaning
When something pulls your focus away from actually flying the airplane, and your control of the aircraft suffers because of it.
Full Definition
A loss of focus on the primary task of flying the airplane caused by shifting attention to a secondary task, distraction, or unexpected event, resulting in degraded aircraft control, situational awareness, or both.
Context Anchor
Seen in human factors discussions about distractions, cockpit workload, and loss of aircraft control.
Derivation
From Latin divertere, 'to turn aside.' A diversion of attention is literally attention being turned aside from where it should be — flying the airplane.