Plain English Meaning
Noticing — quickly and correctly — that the wing is about to stop producing enough lift to keep flying. The pilot picks this up from how the airplane feels, sounds, and responds, not just from one single cue.
Full Definition
The pilot's ability to identify that the airplane is approaching or has entered an aerodynamic stall, based on a combination of cues such as buffeting, reduced control effectiveness, decay in pitch authority, mushy controls, sluggish response, audible or visual stall warning, and a high pitch attitude relative to the flight path.