Plain English Meaning

The line around the Earth where a compass needle lies perfectly flat instead of tilting toward one of the magnetic poles. Stand on it and the magnetic pull is purely sideways, with no downward pull on either end of the needle.

Full Definition

An imaginary line around the Earth, near but not identical to the geographic equator, where the Earth's magnetic field runs horizontally and a freely suspended magnetic needle has zero dip (no tendency to tilt up or down). North of this line the needle dips downward toward the north magnetic pole; south of it the needle dips downward toward the south magnetic pole.

Context Anchor

Seen in magnetic compass and dip error discussions, especially when explaining why compass errors change with magnetic latitude.

Derivation

From Latin 'magnes' (the magnetic stone, lodestone) and 'aequator' (one who makes equal). The geographic equator is the line that 'equalizes' the two hemispheres; the magnetic equator does the same job for the magnetic field — it is the line where the pull toward each magnetic pole is balanced.