black tie

plural black ties

1. countableA black bow tie worn with a dinner jacket.
2. uncountableEvening 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.

   You are cordially invited to the Social Club's tenth Anniversary Celebration of its Brick-and-Mortar Building. Dress: Black Tie and White Tie are encouraged, but optional.

3. countableuncountableUsed other than figuratively or idiomatically: see black, tie.