The Collaborative International Dictionary of English v.0.48:

formalwear \formalwear\ n.
   attire to wear on formal occasions in the evening.

   Syn: eveningwear, evening dress, evening clothes.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

formalwear
    n 1: attire to wear on formal occasions in the evening [syn:
         formalwear, eveningwear, evening dress, evening
         clothes]