ab- is a prefix — a small piece added to the front of a word to change its meaning. It comes from Latin and carries the basic sense of “away from,” “off,” or “out of.” When you see “ab-” at the start of a word, it often signals movement or direction away from something.

How it’s used

In medicine and everyday English alike, “ab-” tends to mark something moving away, departing from the normal, or separating from a center. A few clear examples:

• abduction → moving a body part away from the midline of the body (e.g., raising your arm out to the side). Its opposite is adduction, moving toward the midline.
• abnormal → away from what is normal or typical.
• absent → away, not present.
• abduct → to lead or carry away.
• aberrant → wandering away from the usual or expected path.

A useful contrast

It helps to compare “ab-” with its near-opposite, ad- (“toward”):

• abduction = movement away from the body’s center
• adduction = movement toward the body’s center

Just one letter changes the direction completely — which is exactly why these prefixes matter so much in anatomy and medicine, where precise direction can change the entire meaning of a term.

Why prefixes like this are worth knowing

Once you recognize that “ab-” means “away from,” you can often work out the rough meaning of an unfamiliar word without a dictionary. This is especially handy in medical and scientific vocabulary, where the same Latin and Greek building blocks appear over and over again.