A "ribbon sash" is a wide decorative strip of cloth, usually made of ribbon material, that is worn around the waist to add color or style to a dress or outfit. Full definition
Ivory Beaded Lace with Satin Ribbon Sash, Bridal Ivory Pearl Sash, Bridesmaid Sash, Champagne Pearl Lace Sash Belt, Pearl Wedding Sash - 1818 (etsy.com)
Zoey Deutch: While Zoey also wore a monochromatic colour palette, her cream gown struck a much more dramatic chord with its asymmetric, black ribbon sash. (redcarpet-fashionawards.com)
The rose ribbon sash adds a pop of pink at the waist, but can easily be removed and replaced with a sash of your choosing. (davidsbridal.com)