The Collaborative International Dictionary of English v.0.48:

D'ecollet'e \D['e]`col`le*t['e]"\ (d[asl]`k[o^]l`le*t[asl]"), a.
   [F., p. p. of d['e]colleter to bare the neck and shoulders;
   d['e]- + collet collar, fr. L. collum neck.]
   1. Leaving the neck and shoulders uncovered; cut low in the
      neck, or low-necked, as a dress.
      [1913 Webster]

   2. Wearing a d['e]collet['e] gown.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

decollete
    adj 1: (of a garment) having a low-cut neckline; "a low-cut
           neckline" [syn: decollete, low-cut, low-necked]