The Collaborative International Dictionary of English v.0.48:

D'ecolletage \D['e]`col`le*tage"\ (d[=a]`k[-o]`l'*t[.a]zh), n.
   [F. See D['e]collet['e].] (Costume)
   1. The upper border or part of a low-cut (i.e.,
      d['e]collet['e]) dress.
      [Webster 1913 Suppl. +PJC]

   2. The exposed upper parts of the breasts of a woman wearing
      a low-cut dress; as, transfixed by her stunning
      decolletage.
      [PJC]

WordNet (r) 3.0 (2006):

decolletage
    n 1: a low-cut neckline on a woman's dress