Plain English Meaning
An instrument that shows which direction the airplane is pointing. It uses a spinning gyro to give a steady reading, but the pilot has to set it to match the compass and reset it now and then because it slowly drifts.
Full Definition
A gyroscopic flight instrument that displays the airplane's heading relative to a manually set reference, typically magnetic north. Because the gyro itself does not seek north, the pilot must periodically align it with the magnetic compass to correct for gyroscopic drift.
Context Anchor
Seen on the instrument panel during taxi, before takeoff, and in flight when the pilot needs a clear heading reference.