(1) A substance that renders another substance (e.g., a catalyst) active, or that accelerates a process or reaction. (2) The fragment produced by chemical cleavage of a proactivator, that induces the enzymic activity of another substance. (3) An apparatus for making substances radioactive (e.g., a neutron generator or a cyclotron). (4) A removable type of myofunctional orthodontic appliance that acts as a passive transmitter of force, produced by the function of the activated muscles, to the teeth and alveolar process that are in contact with it. (5) A protein that binds to a DNA sequence before RNA polymerase transcription, sometimes referred to as coactivator.