Alternatively referred to as the arithmetic mean, average is the sum of a series of numbers, divided by the total amount of numbers. For example, suppose we have the following series of numbers: 1, 2, 3, 4, 1, 2, and 3. The sum of these numbers is 16, 16 divided by 7 is 2.28, therefore, 2.28 is the average of these numbers.
In Microsoft Excel and other spreadsheets, you can determine the average of multiple cells by using the average formula function. For example, typing in =AVERAGE(A1:F1) would determine the average of cells A1 through F1.