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.

Source: Webster's Revised Unabridged Dictionary (1913)