Plain English Meaning
The tail of most airplanes pushes downward in flight. That downward push is what holds the nose up and keeps the airplane balanced front-to-back.
Full Definition
A downward aerodynamic force produced by the horizontal stabilizer (or stabilator) at the tail of a conventional airplane that balances the nose-down pitching tendency caused by the center of gravity being located ahead of the center of lift. This downforce keeps the aircraft in stable longitudinal trim during normal flight.
Context Anchor
Seen in empennage, stability, trim, and weight-and-balance discussions, especially when explaining why the often produces a .