Plain English Meaning

The airplane is flying at a steady speed in a straight line, with no change in how fast it's going or which way it's headed.

Full Definition

A condition of flight in which the aircraft is neither speeding up, slowing down, nor changing direction. All forces acting on the aircraft — lift, weight, thrust, and drag — are in balance, so velocity (both speed and direction) remains constant.

Context Anchor

Seen in straight-and-level flight, instrument flying, and discussions of how the flight instruments behave when the airplane is stabilized.

Derivation

From 'un-' (not) plus 'accelerated' (from Latin accelerare, 'to hasten'). In , means any change in velocity — including speed or direction. So 'unaccelerated' means no change in either.