The Collaborative International Dictionary of English v.0.48:

nightrobe \night"robe`\, night-robe \night"-robe`\n.
   1. A loose dress designed to be worn in bed by women; a
      nightgown.

   Syn: nightgown, nightie, nightdress.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

night-robe
    n 1: lingerie consisting of a loose dress designed to be worn in
         bed by women [syn: nightgown, gown, nightie, night-
         robe, nightdress]