Plain English Meaning
Your inner ear, which helps you balance, can lie to you in flight. When you can't see the horizon, it may tell you the airplane is doing one thing when it's actually doing another.
Full Definition
False sensations of motion, attitude, or position generated by the inner ear's balance organs (the vestibular system) when normal visual references are lost, such as in clouds or at night. These illusions occur because the inner ear can misinterpret the accelerations and angular movements experienced in flight, leading the pilot to feel a turn, climb, descent, or bank that is not actually happening.