black tie

Short definition from Wiktionary

Noun

1. 1
   A black bow tie worn with a dinner jacket.
2. 2
   Evening dress; a standard of dress which is less formal than white tie, consisting of black dinner jacket or tuxedo jacket, and matching trousers, white shirt and black bow tie or, possibly, military dress or national costume.
3. 3
   Used other than figuratively or idiomatically: see black, tie.

Adjective

1. 1
   Moderately formal; usually requiring a dinner jacket (tuxedo) for men.