The Collaborative International Dictionary of English v.0.48:

black-tie \black-tie\ adj.
   requiring semiformal evening clothes, e. g. a black bowtie
   and a tuxedo or dinner jacket for men, and a formal dress for
   women; contrasted with white-tie, for a fully formal
   occasion, and with informal, and casual. a black-tie
   dinner

   Syn: semi-formal, semiformal.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

black-tie
    adj 1: moderately formal; requiring a dinner jacket; "he wore
           semiformal attire"; "a black-tie dinner" [syn:
           semiformal, semi-formal, black-tie]