ab- (also abs- before certain consonants) is a particle, most often a prefix, that occurs in many languages. In Latin it was both a prefix and a preposition meaning "off, away (from), departing from, outside of" and expressed the idea of removal, absence, separation, or a lower value. It first appeared in English in the Middle English period in words borrowed from Old French, and continued to be borrowed in Modern French words and eventually from Classical Latin, especially in the vocabulary of science, medicine, botany, and so on.

The prefix ab- (short for absolute) is also used to indicate "absolute" in cgs units used in electromagnetism, such as abhenry for inductance, abohm for resistance and abvolt for potential.

ab- in Interlingua is a productive affix used much as in English, and most English words that begin with ab- also occur in Interlingua: absolute, abolition, abrupte. Occasionally, ab also appears as a preposition, meaning "from, away from".