Real Dictionary

black tie
A black tie event is a formal social event such as a party at which people wear formal clothes called evening dress.

black tie in British English
a black bow tie, properly worn with a tuxedo
adjective: involving formal evening dress for men, including a dinner jacket or tuxedo and a bow tie
noun: a black bow tie worn with a dinner jacket
noun: a black bow tie, worn with a dinner jacket
noun: formal evening dress for men, including a dinner jacket or tuxedo and a bow tie

black-tie in American English
adjective: requiring that guests wear formal attire, esp. that men wear black bow ties with tuxedos or dinner jackets