A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals, an excursion is not a "permanent" re-orientation of the large-scale field, but rather represents a dramatic, typically a (geologically) short-lived change in field intensity, with a variation in pole orientation of up to 45° from the previous position.

Excursion events typically only last a few thousand to a few tens of thousands of years, and often involve declines in field strength to between 0 and 20% of normal. Unlike full reversals, excursions are generally not recorded around the entire globe. This is certainly due in part to them not registering well in the sedimentary record, but it also seems likely that excursions may not typically extend through the entire global geomagnetic field. There are significant exceptions, however.