The Collaborative International Dictionary of English v.0.48:

nightie \nightie\ n.
   1. Any loose-fitting clothing for wear in bed.
      [PJC]

   2. Especially: a loose dress designed to be worn in bed by
      women; a nightgown.

   Syn: nightgown, night-robe, nightdress.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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