Plain English Meaning
An instrument on the panel that shows whether the nose is pointing up, down, or level, and whether the wings are level or tilted. It does this by displaying a small airplane against a moving horizon, so the pilot can see the airplane's position even when the real horizon is not visible outside.
Full Definition
A gyroscopic flight instrument that displays the airplane's pitch (nose up or down) and bank (wings level or tilted) relative to the natural horizon. It presents this information through a miniature airplane symbol fixed to the instrument case, set against a moving background that represents the sky and ground separated by a horizon line.