Plain English Meaning
The slowest you can fly while still holding altitude. Go any slower and the airplane will start to sink or stall.
Full Definition
The slowest airspeed at which an airplane can sustain level flight at a given power setting and configuration, where the wing is producing just enough lift to balance weight and any further reduction in speed will result in a loss of altitude or a stall.
Context Anchor
Seen when learning slow flight, elevator control, and how pitch changes the airplane’s energy state.