The Collaborative International Dictionary of English v.0.48:

Rabbet \Rab"bet\, n. [See Rabbet, v., and cf. Rebate, n.]
   [1913 Webster]
   1. (Carp.) A longitudinal channel, groove, or recess cut out
      of the edge or face of any body; especially, one intended
      to receive another member, so as to break or cover the
      joint, or more easily to hold the members in place; thus,
      the groove cut for a panel, for a pane of glass, or for a
      door, is a rabbet, or rebate.
      [1913 Webster]

   2. Same as Rabbet joint, below.
      [1913 Webster]

   Rabbet joint (Carp.), a joint formed by fitting together
      rabbeted boards or timbers; -- called also rabbet.

   Rabbet plane, a joiner's plane for cutting a rabbet.
      --Moxon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rabbet joint
    n 1: a joint formed by fitting together two rabbeted boards