This pizza is divided into #5# slices of equal size.
Each slice is one-fifth of the pizza. We write this as #\tfrac 15#.

Someone eats #4# of the #5# slices.
To denote #4# out of #5#, we write #\tfrac45#.

The number #\require{color} \definecolor{blue}{RGB}{45, 112, 179}\tfrac{\orange{4}}{\blue{5}}# is called a fraction.
In the fraction #\tfrac{\orange{4}}{\blue{5}}#, #\orange{4}# is called the numerator.
In the fraction #\tfrac{\orange{4}}{\blue{5}}#, #\blue{5}# is called the denominator.

The numerator and the denominator are separated by a line, which does not have a special name in English.