white tie
noun
Definitions
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.
A dress code specifying white tie for men and ball gown or evening dress for women, or, alternatively, full military dress or national costume.
Example: You are cordially invited to dinner with His Excellency the President. Dress: White Tie.