What do the British call the item of clothing that Americans know as a 'vest'?

A waistcoat (UK and Commonwealth; colloquially called a weskit), or vest (US and Canada), is a sleeveless upper-body garment. It is usually worn over a dress shirt and necktie and below a coat as a part of most men's formal wear. It is also sported as the third piece in the traditional three-piece male suit.

Various types of waistcoats may have been worn in theatrical manners such as performances and masquerades prior to what is said to be the early origins of the vest. During the 17th century, the forerunner to the three-piece suit was appropriated from the traditional dress of diverse Eastern European and Islamic countries.

The justacorps frock coat was copied from the long zupans worn in Poland and the Ukraine, the necktie or cravat was derived from a scarf worn by Croatian mercenaries fighting for King Louis XIII of France, and the brightly coloured silk waistcoats popularised by King Charles II of England were inspired by exotic Indian and Persian attire acquired by wealthy English travellers.