Plain English Meaning
Stopping a climb or descent and settling the airplane into steady flight at the altitude you want to hold.
Full Definition
The transition maneuver from a climb or descent to level flight at a chosen altitude, accomplished by smoothly adjusting pitch attitude to stop the vertical movement and then setting power, trim, and airspeed for sustained level cruise.
Context Anchor
Seen during climb training, descent training, and any time a pilot is approaching an assigned or chosen altitude.
Why Pilots Care
A properly timed level-off prevents altitude overshoot, maintains airspeed, and produces a smooth to without abrupt or changes.