Abductor muscle: Any muscle used to pull a body part away from the midline of the body. For example, the abductor muscles of the legs spread the legs away from the midline and away from one another.
The word "abductor" comes from the Latin prefix "ab-" meaning "away from" + "ducere" meaning "to draw or lead" = "to draw away from."
The prefix "ab-" is opposed to the prefix "ad-". Therefore, the opposite of "abductor" is "adductor." An abductor muscle opposes an adductor muscle.