What is this clothing accessory called?

A cummerbund is a broad waist sash that is usually pleated, which is often worn with single-breasted dinner jackets or tuxedos. The cummerbund originated in Iran and was adopted by British military officers in colonial India, where they saw it worn by Indian men. It was adopted as an alternative to a waistcoat, and later spread to civilian use.

The modern use of the cummerbund is as a component of a traditional black tie event. The word cummerbund is the Anglicized form of the Persian word 'kamarband', the name commonly used in the South and West Asia including India for the article of clothing. It entered English vocabulary in 1616 from India. It is a combination of the Hindi words ('kamar') meaning 'waist' and ('band') meaning 'strap' or 'lacing'.

The form of the cummerbund is a wide band around the waist, and its origin as part of black tie determined the acceptable colors. Once it was adopted as civilian dress, beginning as a largely summer option with informal dinner jackets, such as Burmese fawn and white, it was restricted to the narrow range of colors which accompany black tie. Cummerbunds were predominantly black, sometimes midnight blue to match the trousers, and occasionally maroon. The pleats face up because they were originally used to hold ticket stubs and similar items, explaining the slang name 'crumb-catcher'.