Denominator

The denominator is the bottom part of a fraction.
For example, in the fraction 3/4, the denominator is 4.

3
4 <=== denominator

The number on the top is called the numerator. The denominator is the number of parts that you cut something into. For example, if you cut a pizza into 4 pieces (the denominator), and you get 3 pieces (the numerator), then you got 3/4 of the pizza.

Now, here's a question for you:

In the fraction 7/13, what is the denominator?