mid-14c., encresen, "become greater in size or number" (intransitive); late 14c., "cause to grow, enlarge" (transitive), from Anglo-French encress-, Old French encreiss-, present participle stem of encreistre, from Latin increscere "to increase, to grow upon, grow over, swell, grow into," from in- "in" (from PIE root *en "in") + crescere "to grow" (from PIE root *ker- (2) "to grow"). Modern English restored the Latin spelling 16c. Related: Increased; increasing.
late 14c., "action of increasing; results of an increasing," from increase (v.) or from verbs formed from the noun in Old French or Anglo-French. The stress shifted from 18c. to distinguish it from the verb.