Plain English Meaning
The 'seat of the pants' sense -- the feel you get from your skin, muscles, and joints that tells your brain how your body is positioned and which way is down. It works fine on the ground but can mislead you in flight because the forces from turning, climbing, or accelerating feel similar to gravity.
Full Definition
The body's system of nerve endings in the skin, joints, and muscles that sense pressure, stretch, and position, and feed that information to the brain to help judge orientation, balance, and movement. In flight, it is one of three orientation systems pilots rely on, alongside vision and the vestibular (inner ear) system, and it is easily fooled because it cannot distinguish between the pull of gravity and the pull of acceleration.
Context Anchor
Seen in when why body feelings, sometimes called “” cues, cannot be trusted by themselves.