late 14c., a term in astrology (from French équation, 14c.); general sense of "action of making equal" is from 1650s, from Latin aequationem (nominative aequatio) "an equal distribution, a sharing in common," noun of state from past participle stem of aequare (see equal, adj.). Mathematical sense is from 1560s, on notion of equalizing the expressions; Chemistry sense is from 1807.