Simple Definition:

Mean, in math, refers to the average of a set of numbers. [Average is the sum of all the numbers divided by the total count of numbers.]

For example, if we have the numbers 2, 4, 6, 8, and 10, the mean of these numbers would be (2 + 4 + 6 + 8 + 10) / 5 = 6.

Very Simple Definition:

In math, mean is the number you get when you add up a group of numbers and then divide the sum by how many numbers there are.

For example, if you have the numbers 1, 2, and 3, the mean is (1 + 2 + 3) / 3 = 2.