A prefix of Old English origin meaning "against", "back", "in return", "away", represented in Modern English by a-, an-, on-, and in altered form by the reverse-action prefix un- (e.g. unbuckle). Also as the initial letter d in dread (< Old English ondrǣdan).
over, above
excessive
existing in more than three spatial dimensions
linked non-sequentially
beyond
intensely, extremely, or exceptional