white tie

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

• noun A white bow tie worn as a part of men's formal evening dress.
• noun Men's formal evening dress.

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

• noun The most formal kind of eveningwear, usually worn on state occasions and the like, consisting of an evening tailcoat, trousers, white shirt, white bow tie and waistcoat with sash, alternatively, full military dress, or national costume.