A cummerbund is a sash that is part of a tuxedo. The cummerbund is worn around the waist, and its color usually matches the color of one's tie. You might never see one again after your prom.
If you’ve ever worn or seen a tuxedo, you’ve probably noticed the jacket, tie, and some other material around the waist area: that’s the cummerbund. A cummerbund is a type of sash that's broad and pleated. Some sashes are worn over the shoulder, but a cummerbund goes around the waist. Though cummerbunds are now associated with higher classes of people, they were originally worn by less privileged classes in India. The word is from the Hindi for "waistband."