Definition from Wiktionary, the free dictionary
black tie (uncountable)
- 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.
- Used other than as an idiom: see black, tie.
- (standard of dress): business casual, casual, dress casual, formal, informal, military dress, morning dress, national costume, white tie