The piece of wood that is glued to the guitar top behind the soundhole.  It has a slot that the saddle sits in, and provides a hard surface for the bridge pins.  The bridge is usually rosewood or ebony.  The bridge is the black strip of wood in the picture below.

{{http://www.rhythmstrummer.com/images/guitarbridge.jpg}}\\
photo by [[http://www.flickr.com/photos/matsuyuki/|matsuyuki]]