black tie

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A black bow tie worn with a tuxedo.
• noun Semiformal evening wear typically for men, usually requiring a tuxedo.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Evening 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.
• noun Used other than as an idiom: see black,‎ tie.