The Collaborative International Dictionary of English v.0.48:

circuit breaker \cir"cuit break"er\, n. (Elec.)
   A device contained within an electrical circuit designed to
   interrupt the circuit when the current exceeds a preset
   value; it is sometimes called a contact breaker. Its
   function is to prevent fire or damage to the circuit or the
   devices on the circuit which could be caused by excess
   current, such as that caused by a short-circuit. It differs
   from a fuse in not having a conducting element which melts
   (see 2nd fuse, n.). Circuit breakers are designed to be
   easily reset, i. e. to reclose the circuit after it has been
   opened by the circuit breaker; this is usually accomplished
   by simply moving a switch back and forth. They are commonly
   used in buildings to protect individual electrical lines, and
   are often contained in groups within a box called a circuit
   breaker panel, which divides the current from a main
   electrical line into multiple circuits each protected by a
   separate circuit breaker. Less commonly they may be found on
   individual devices.
   [PJC]

WordNet (r) 3.0 (2006):

circuit breaker
    n 1: a device that trips like a switch and opens the circuit
         when overloaded [syn: circuit breaker, breaker]