de rigueur is a French expression that literally means "of rigor" or "of strictness". In English language usage, it means, "necessary according to etiquette, protocol or fashion."
Something that is de rigueur is required by convention or fashion, but not by any hard rule. For example, among the upper classes during the Victorian era, dressing for dinner was de rigueur.
"De rigueur" is also a type of standard similar to a de facto standard, though de facto refers to a practice that is practically law.