From Longman Dictionary of Contemporary Englishblack-tieˌblack-ˈtie adjective DCCa black-tie event is one at which people wear special formal clothes, such as tuxedos for men → white-tie
Examples from the Corpus
black-tie• About 1,500 people at the black-tie affair greeted Clinton with an ovation as he approached the podium.• Evening meals were black-tie affairs attended by selected officers.• The Democratic mayor has thrown block parties for the masses and black-tie affairs for the elite.• a black-tie dinner• Marshall set only one condition on his gift: No black-tie dinners in his honor.• A tent worthy of Barnum & Bailey contained an assortment of black-tie outfits worthy of Fortune magazine.• So the entire black-tie party for 1,500. 