Plain English Meaning
It's when your body tells you the airplane is doing one thing, but it's actually doing another — for example, feeling like you're flying straight and level when you're really in a turn or a descent.
Full Definition
A condition in which a pilot loses an accurate sense of the airplane's position, attitude, or motion relative to the earth, typically because the body's sensory cues (inner ear, muscles, and vision) conflict with what the flight instruments or outside references actually show.
Context Anchor
Seen when discussing the feel of the airplane, especially the limits of trusting body sensations instead of outside visual references and flight instruments.