black tie
1. adjective Of, pertaining to, or requiring formal evening dress for men, typically consisting of a tuxedo or dinner jacket with a tie. Often hyphenated. I'm having a black-tie party for my 30th birthday, so don't show up in jeans and a T-shirt! The gala will be strictly black tie: no guests will be admitted without proper attire.
2. noun Formal evening dress code for men, typically consisting of a black tuxedo or dinner jacket with matching trousers and bow tie. The gentlemen, all of them in black tie, posed for the publicity photograph.