The Collaborative International Dictionary
Epi- \Ep"i-\ [Gr. 'epi` on, upon, to; akin to Skr. api besides, and prob. to L. ob to, before, on account of, and perh. to E. of, off.] A prefix, meaning upon, beside, among, on the outside, above, over. It becomes ep-before a vowel, as in epoch, and eph-before a Greek aspirate, as in ephemeral.
Douglas Harper's Etymology Dictionary
before vowels reduced to ep-, before aspirated vowels eph-, 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" (cognates: 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"). A productive prefix in Greek; also used in modern scientific compounds (such as epicenter).
pre. above, over, on, in addition to.