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):

nightclothes
    n 1: garments designed to be worn in bed [syn: nightwear,
         sleepwear, nightclothes]