Real Dictionary

epi- in American English
prefix: a prefix occurring in loanwords from Greek, where it meant “upon,” “on,” “over,” “near,” “at,” “before,” “after” (epicedium; epidermis; epigene; epitome). On this model, epi- is used in the formation of new compound words (epicardium; epinephrine)

epi- in British English
on, upon, over, on the outside, anterior, beside, besides, among: it becomes ep- before a vowel and, as eph-, reflects original combining with a word containing an initial aspirate, represented by English h
prefix: on; upon; above; over