A Rabbet, also known as a Rebate is a wood working term for a two sided square groove that has been milled into the edge of a work piece. A rabbet creates a mechanical advantage when used to join two pieces of wood together. A rabbet is not the strongest joint it is often just enough for the task at hand.

window rabbet
A rabbet milled into a window sash is where an 1/8″ thick piece of glass will rest.