Le trait ou barre de fraction ou vinculum signifie que l’on divise le numérateur par le dénominateur.

— Wikipedia: Fraction (mathématiques)

In French, like in English, you can write a fraction horizontally or vertically. You use a barre de fraction (“fraction bar” in English) to separate the numerator from the denominator. When the fraction is written horizontally the barre de fraction is called a barre oblique (“fraction slash” or “fraction stroke” in English).

I’ve studied math and physics for a bunch of years, I’ve heard “barre de fraction” much, much more.