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).
Opposite of, reverse.
Related to antiparticles or antimatter.
Against, hostile to.
Physically opposite: applied to mountain ranges.
Indicating that a game's conventional victory conditions have been reversed: misère.
Counteracting, neutralizing.
Contrasting with the norm.
That reacts with immunoglobins found in the specified animal.