A reactor with a fast, as opposed to thermal, neutron spectrum. Among other things, implies that it must be cooled by something else than plain water, which is is a neutron moderator (that is, slows down the neutrons to thermal velocities). Often the coolant of choice is a metal like sodium or lead.

Fast reactors enables breeding, that is producing more fissile material from the fuel than is consumed. They are also better at burning up various isotopes that thermal reactors have trouble with.