A cummerbund is a pleated waistband worn with a tuxedo and is designed to cover the fabric of the shirt where it meets the waist to maintain the sleek nature of a tuxedo.

Also popular as promwear, the pleats in a cummerbund should face up.

The word is sometimes mispronounced as ‘cum-ber-bund (guilty as charged) .

Quick Cummerbund History and Know-How

The cummerbund is an Indian invention, adopted by the Brits who thought it would be useful for catching breadcrumbs while dining.  (Yes.  You read that correctly). cummerbund makes up a formal tuxedo outfit and is included in the rental package. It should be worn pleats facing up (too catch the breadcrumbs) with a single breasted jacket, bow tie and no vest. Read more: Wedding Guide to Suits: Part VI Cummerbunds | Custom Menswear and Trends

Merriam-Webster Online Dictionary
cummerbund (noun)
a broad waistband usually worn in place of a vest with men's dress clothes and adapted in various styles of women's clothes
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