The Collaborative International Dictionary of English v.0.48:

White-tie \White-tie\, a.
   requiring formal evening clothes, usually interpreted as a
   white bow tie and a tailcoat for men, and a formal evening
   dress for women; -- as, a white-tie reception at the embassy.
   Contrasting with black-tie and informal.
   [PJC]

WordNet (r) 3.0 (2006):

white-tie
    adj 1: requiring white ties and tailcoats for men; "a white-tie
           occasion"