EPI

Ep"i-. Etym: [Gr. api besides, and prob. to L. ob to, before, on account of, and perh. to E. of, off.]

Definition: 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 Unabridged Dictionary 1913 Edition