white tie

Short definition from Wiktionary

Noun

1. 1
   The most formal kind of eveningwear for men, consisting of an evening tailcoat, trousers with strips of braid down the side seams, white stiff-fronted shirt, white bow tie and waistcoat with or without sash.
2. 2
   A dress code specifying white tie for men and ball gown or evening dress for women, or, alternatively, full military dress or national costume.

Adjective

1. 1
   Of an event: very formal; usually requiring a tailcoat for men.