black-tie

from 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}]

from 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]