Definition:Arithmetic Mean

Let $$x_1, x_2, \ldots, x_n \in \mathbb{R}$$ be real numbers.

The arithmetic mean of $$x_1, x_2, \ldots, x_n$$ is defined as:

$$A_n = \frac {\displaystyle\sum_{k=1}^n x_k} {n}$$

That is, to find out the arithmetic mean of a set of numbers, add them all up and divide by how many there are.

Comment
This is the quantity that is usually referred to in natural language as the "average" of a set of numbers.

Note that there are several kinds of "average" and indeed, several different kinds of "mean". This is just one of them.