The Collaborative International Dictionary of English v.0.48:

Reenter \Re*en"ter\, v. i.
   To enter anew or again.
   [1913 Webster]

   Reentering angle, an angle of a polygon pointing inward, as
      a, in the cut.

   Reentering polygon, a polygon having one or more reentering
      angles.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reentering polygon
    n 1: a polygon with one or more reentrant angles [syn:
         reentrant polygon, reentering polygon]