[epi-] or ep- prefix [ME, fr. MF & L; MF, fr. L, fr. Gk, fr. epi on, at, besides, after; akin to OE eofot crime] 1: upon "epiphyte": besides "epiphenomenon": attached to "epididymis": over "epicenter": outer "epiblast": after "epigenesis"
2. a: chemical entity related to (such) another "epimer" b: chemical entity distinguished from (such) another by having a bridge connection "epichlorohydrin"