Source : Webster's Revised Unabridged Dictionary (1913)

Nightgown \Night"gown`\, n.
   A loose gown used for undress; also, a gown used for a
   sleeping garnment.

Source : WordNet®

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