Write a fraction like 1/2 and the denominator is the number on the bottom. In this case, the 2.
Another word for denominator is divisor. Both of these words refer to the number under the line in a common fraction. Similarly, when you're talking about statistical values, a denominator refers to the whole number or population from which samples are taken. The national census, for example, gathers the total number of people and households in the country so that there is a denominator by which to compare statistics like unemployment or welfare.