Plain English Meaning
The direction you plan to fly from one point to another, measured against magnetic north (the north your compass points to) rather than the geographic North Pole.
Full Definition
The intended horizontal direction of flight between two points, measured in degrees clockwise from magnetic north. Magnetic course is the true course corrected for magnetic variation but not for wind drift.
Context Anchor
Seen on IFR flight planning forms, navigation logs, and route-planning figures where each route segment has a planned course.