Plain English Meaning
Flying in a straight line, at one altitude, with the wings level. Not climbing, not descending, not turning.
Full Definition
A flight condition in which the airplane maintains a constant heading and a constant altitude, with the wings level and no turn in progress.
Context Anchor
Used in basic airplane control and slow-flight practice, especially when a pilot is asked to hold direction and altitude while flying slowly.
Why Pilots Care
It is the stable baseline from which all maneuvers begin and the condition to which pilots return to re-establish control and reference speeds.