The Collaborative International Dictionary of English v.0.48:

mousetrap \mouse"trap`\, mouse trap \mouse" trap`\, n.
   Any device that catches, and usually kills, mice. They are of
   various designs, the most common being a stiff loop of wire
   mounted on a small wooden platform base and attached to a
   strong spring, which holds the loop firmly against the base.
   To activate the trap, the loop is pulled through a 180[deg]
   arc against the tension of the spring and held against the
   base by a delicate metal catch, which can keep the loop from
   moving, though in a state of high tension. The metal catch is
   moved when a mouse tries to take a piece of bait attached to
   it, releasing the loop which forcefully moves though an arc,
   usually killing the mouse. A larger version of the same
   device is used as a rat trap.
   [1913 Webster]