Costume de rigueur

The term "costume de rigueur" (from French language) refers to a strict formal dresscode for the attire worn at a special event or in a particular venue. It is commonly used in invitations to a masquerade ball for the Mardi Gras in Mobile (Alabama) or New Orleans Mardi Gras (Louisiana) in the United States. However, the term is also used to describe any rigorous style of attire as worn. As a phrase adapted from French, the term "costume de rigueur" is not italicized in typical usage, although still used in French language.

At a formal masquerade ball during Mardi Gras celebrations in the United States, the stipulation of "costume de rigueur" means that men are required to wear white tie and black tails (with white bow tie), and women should wear a full-length evening gown.

^[p] The term "costume de rigueur" is typically pronounced as "Cos-toom Day Rig-ger" rather than in the formal French style.

...
Wikipedia