a prefix occurring originally in loanwords from Greek, where it was joined to verbs, deverbal forms, and other parts of speech. Among its functions in Greek, apo- has the spatial sense “away, off, apart” (apogee; apocope; apostasy; apostrophe); it occurs with deverbals that denote a response or defense (apodosis; apology) and is found on verbs having perfective force relative to a corresponding simple verb (apoplexy; aposiopesis). In modern scientific coinages in English and other languages, apo- marks things that are detached, separate, or derivative (apocarpous; apoenzyme).
before vowels ap-, word-forming element meaning "from, away from, separate, free from," from Greek apo "from, away from; after; in descent from," in compounds, "from, asunder, away, off; finishing, completing; ceasing from; back again," from PIE root *apo- "off, away" (cf. Sanskrit apa "away from," Avestan apa "away from," Latin ab "away from, from," Gothic af, Old English of "away from").