black tie

1. ADJ A black tie event is a formal social event such as a party at which people wear formal clothes called evening dress. (社交场合)穿晚礼服的; 正式的[usu ADJ n]

...a black-tie dinner for former students. ...老同学聚会的正式晚宴。

2. N-UNCOUNT If a man is dressed in black tie, he is wearing formal evening dress, which includes a dinner jacket or tuxedo and a bow tie. 男式晚礼服

Most of the guests will be wearing black tie. 绝大多数客人都会穿晚礼服。