Plain English Meaning
The single spot on a wing or tail surface where you can think of all the lift as pulling from. As the wing's angle to the air changes, that spot moves forward or backward along the surface.
Full Definition
The point on an airfoil where the total sum of aerodynamic pressure forces is considered to act, producing a single resultant lift vector. Its location along the chord shifts with changes in angle of attack, and on a contaminated or iced airfoil it can move unpredictably, altering pitching moments.
Context Anchor
Seen in aircraft that show lift, tail , or effects with arrows acting on a wing or tail surface.