- That which reacts with something else, or that responds to an action or stimulus.
- A device which utilizes controlled nuclear fusion or fission to generate energy. A fission reactor, for instance, has the following key components: a fissionable fuel, such as uranium-235, a moderator, such as heavy water, shielding, such as that provided by thick concrete, control rods, such as those made with boron, and a coolant such as water. The heat produced by the nuclear reactions can be utilized to generate electricity. Also called nuclear reactor, or atomic reactor.
- A component, circuit, or device which introduces a reactance to a circuit. Capacitors and inductors are examples.