The Collaborative International Dictionary of English v.0.48:

Nightgown \Night"gown`\, n.
   A loose gown used for undress; also, a gown used for a
   sleeping garment.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

nightclothes \nightclothes\ n.
   garments designed to be worn in bed, such as pyjamas, a
   nightgown, etc.

   Syn: nightwear.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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