Plain English Meaning
Everything pushing and pulling on the airplane cancels out, so it keeps doing what it's already doing without speeding up, slowing down, climbing, or descending.
Full Definition
A condition in which all forces and moments acting on the airplane are balanced, producing no acceleration. The airplane maintains a steady state — constant airspeed, constant altitude, and constant attitude — because thrust equals drag and lift equals weight.
Context Anchor
Seen in constant-speed propeller discussions when the handbook explains how the propeller governor holds a selected RPM.