Definition
A small placard mounted next to the magnetic compass that shows the corrections a pilot must apply to the compass reading to account for deviation — the magnetic error caused by metal components and electrical equipment in the airplane itself. The card lists the heading the pilot should fly (steer) to actually achieve a desired magnetic heading, typically in 30-degree increments.
Plain English
A little chart in the cockpit, right by the compass, that tells you what heading to actually steer to end up going the direction you want. It corrects for the small errors the airplane's own metal and wiring introduce into the compass.
Context Anchor
Found in the cockpit near the magnetic compass, especially during preflight checks and heading reference checks in flight.
Derivation
Deviation comes from the Latin deviare, meaning 'to turn away from the path.' Here it refers to the compass needle being pulled away from its true magnetic path by the airplane's own metal and electrical fields.
Why Pilots Care
Without applying the corrections on the card, compass headings will be inaccurate, leading to navigation errors especially during dead-reckoning or when other navigation aids are unavailable.
Intuition Check
Deviation does not mean the airplane has wandered off course. Here it means the compass itself has a known error in that specific aircraft.
Example Sentence 1
Before takeoff, the pilot glanced at the compass deviation card and noted that to fly a magnetic heading of 180, she needed to steer 182 on the compass.
Example Sentence 2
During the preflight inspection the student confirmed that the compass deviation card matched the current equipment configuration in the airplane.