What term describes the angular difference between true north and magnetic north?

The term that best describes the angular difference between true north and magnetic north is magnetic declination. This refers specifically to the angle in degrees, measured east or west from true north to magnetic north. Magnetic declination varies depending on your geographic location and changes over time due to shifts in the Earth's magnetic field.

While magnetic variation is a term that may refer to differences in magnetic readings, it is less commonly used in the specific context of the angular difference between true north and magnetic north. True bearing relates to the direction to a point relative to true north, and course deviation refers to the difference between the intended aircraft course and the actual flight path. Therefore, magnetic declination is the precise term that captures the difference sought in the question.