Plain English Meaning
It's a way of writing down magnetic variation that includes the year the value was good for. Earth's magnetism drifts, so charts label the variation with the year it was measured against, and that data gets refreshed every few years.
Full Definition
A method of expressing magnetic variation values on aeronautical charts and in navigation databases that ties each value to a specific reference year (the epoch year) for which the magnetic variation was calculated. Because Earth's magnetic field shifts over time, magnetic variation data is periodically recalculated and republished against a new epoch year to keep navigation accurate.
Context Anchor
Seen in instrument discussions where explains how magnetic variation is published and updated.