Plain English Meaning
Flying in a straight line at a steady height, with the wings level and the nose held so the airplane stays at the same altitude.
Full Definition
Flight in which a constant heading and a constant altitude are maintained. The wings are held level (no bank) and the pitch attitude is set so the airplane neither climbs nor descends.
Context Anchor
Used early in flight training as one of the basic airplane control skills before turns, climbs, descents, and other maneuvers.