Pronunciation: (ak'tu-vā"tur), [key]
1. a person or thing that activates.
2. Chem., Biochem.a catalyst.
3. any impurity in a mineral that causes luminescence. Cf. inhibitor (def. 3).
4. Orthodontics.a removable appliance, usually of hard plastic, that is worn in the mouth instead of a fixed appliance to help align the teeth and improve the relationship of the lower jaw to the upper.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.