before vowels oneir-, word-forming element meaning "of or pertaining to a dream or dreams," from Greek oneiros "a dream," a word of uncertain origin, perhaps related to Armenian anurj, Albanian (Gheg) âdërrë.
also homoeo-, word-forming element meaning "similar to," Latinized from Greek homio-, from homoios "like, resembling, of the same kind; equal," related to or an expanded form of homos "one and the same" (from PIE root *sem- (1) "one; as one, together with").
word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."
Compounds in classical Greek show the range of the word there: Heterokretes "true Cretan," (that is, of the old stock); heteroglossos "of foreign language;" heterozelos "zealous for one side;" heterotropos "of a different sort or fashion," literally "turning the other way;" heterophron "raving," literally "of other mind."