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1. A substance in the body that converts an inactive molecule into an active agent, such as the conversion of pepsinogen into pepsin by hydrogen ions.
2. A substance that specifically induces an activity, such as an inductor or organizer in embryonic development or in a trophic hormone.
3. A removable orthodontic appliance that transmits force passively from muscles to the teeth and alveolar process in contact with it. Also called myofunctional appliance.

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