Is an aircraft's compass based on magnetic or true north?

Magnetic - as already stated (same is for ships).

There are few exceptions though. When operating polar flights (flights above 78°NL) you want to switch directional reference to True (chances of aggressive autopilot maneuvers). Text weather reports indicate True heading as well.

All compasses show magnetic directions.

The simulated compasses that some GPS devices can display can be set to show directions based on true north.

Navigational directions are based on true bearings - with one exception: airport runway designations are based on magnetic north. (E.G.: Runway 5 is lined up at 50 degrees, magnetic.)

All compasses are MAGNETIC. New or old. Traditional navigation is based upon converting true heading determined from a navigation chart to a magnetic heading that can be maintained using a compass.

Always Magnetic

You steer magnetic headings, even modern gyroscopic and flux compasses are set to read magnetic