Plain English Meaning
An instrument on the panel that shows which way the airplane's nose is pointing, in degrees around a circle. The pilot uses it instead of the magnetic compass because it is steady and easy to read in flight.
Full Definition
A flight instrument that displays the airplane's heading — the direction the nose is pointing — referenced to a compass rose graduated in degrees from 0 through 360. It is gyroscopically driven (mechanically or electrically) and must be periodically aligned with the magnetic compass, because the gyro itself does not seek north.
Context Anchor
Seen during basic flight by reference to instruments, instrument cross-check, and any time an instructor or controller asks you to hold or turn to a specific heading.