Plain English Meaning
A small electronic sensor that detects which way the Earth's magnetic field is pointing, so the aircraft's instruments can show an accurate magnetic heading.
Full Definition
An instrument that measures the strength and direction of a magnetic field. In modern aircraft, a magnetometer senses the Earth's magnetic field and sends that data electronically to the heading system, which displays magnetic heading on the cockpit instruments.
Context Anchor
Seen in compass-turn and heading-system discussions, especially when an aircraft uses an electronic heading display instead of only a simple magnetic compass.
Derivation
From the Greek 'magnes' (magnet) and 'metron' (measure). Literally a 'magnet measurer' -- a device built to measure rather than just along one like a simple compass .