Plain English Meaning

It is the magnetic variation that was used to set up a VOR station when it was last calibrated. The station keeps broadcasting based on that old setting, even if the Earth's magnetic field has shifted since then.

Full Definition

The angular difference between true north and the magnetic north reference that a VOR (or other ground-based navigation station) was aligned to at the time it was last calibrated. It is the magnetic variation value physically built into that station's transmitted radials, regardless of what the current magnetic variation in the area happens to be.

Context Anchor

Seen in instrument procedure design and discussions of magnetic variation, especially when explaining why a published or may not match the current exactly.