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. [1913 Webster]
epi-, prefix (usu. ep- before a vowel or h)
1 upon (epicycle).
2 above (epicotyl).
3 in addition (epiphenomenon).
Etymology:
Gk epi (prep.)