Plain English Meaning

A short summary of how the magnetic compass works and why it matters: it's a simple, standalone instrument that points toward magnetic north using small magnets floating in fluid, and it keeps working even when everything else fails.

Full Definition

A general description of the magnetic compass as a self-contained, liquid-filled instrument that uses small magnets aligned with Earth's magnetic field to indicate the aircraft's heading relative to magnetic north. It is the only direction-indicating instrument in most aircraft that requires no electrical power, no vacuum, and no input from any other system, which is why it serves as the primary backup heading reference.

Context Anchor

Seen in when the and how a gets direction information if other are unavailable or being checked.