word-forming element meaning "on, upon, above," also "in addition to; toward, among," from Greek epi "upon, at, close upon (in space or time), on the occasion of, in addition," from PIE *epi, *opi "near, at, against" (cf. Sanskrit api "also, besides;" Avestan aipi "also, to, toward;" Armenian ev "also, and;" Latin ob "toward, against, in the way of;" Oscan op, Greek opi- "behind;" Hittite appizzis "younger;" Lithuanian ap- "about, near;" Old Church Slavonic ob "on"). Before unaspirated vowels, reduced to ep-; before aspirated vowels, eph-. A productive prefix in Greek; also used in modern scientific compounds (e.g. epicenter).