Plain English Meaning
The airspeed has stopped changing and is holding steady at one value. The needle is no longer creeping faster or slower.
Full Definition
An airspeed that has settled at a steady, unchanging value, where the airspeed indicator needle is no longer trending up or down. It indicates that the aircraft has reached equilibrium between thrust, drag, pitch attitude, and power for the current flight condition.
Context Anchor
Seen in instrument flying when using trend indicators to judge whether the aircraft’s speed is still changing or has settled.