The term Windsor uniform is the name of a type of dress worn by male members of the Royal Family (and very senior courtiers) at Windsor Castle.
The uniform was introduced by George III in 1779. The full dress, which had a good deal of gold braid about it, did not survive beyond 1936, but the undress form is still worn today: a dark blue jacket with red at the collar and cuffs. It is now worn only at Windsor Castle, and since the reign of King Edward VII has generally been worn only as evening dress (though the present Prince of Wales has worn a version of it as a riding coat).
The uniform takes the form of an evening tail coat of dark blue cloth, lapelled, with scarlet collar and cuffs. There are three buttons on each front two at the back of the waist, and two at the end of each tail, and also two on each cuff and one above. The gilt buttons bear a design of a Garter star within a garter, surmounted by the imperial crown. It is worn with a white single-breasted waistcoat with three small gilt buttons of the same pattern, and with plain black evening-dress trousers. When the court is in mourning a black waistcoat and black armband are worn. As well as the tail coat version, the Duke of Edinburgh and the Prince of Wales also wear a dinner jacket version of the coat.