The mean value1 of a set of data is the sum of each datapoint divided by the number of datapoints.

Given a set of $N$ datapoints $X = {x_1, ..., x_N}$, the mean $\mu$ of $X$ is

$$ \mu = \frac{1}{N}\sum_{i=1}^Nx_i .$$

1

Or more specifically, the arithmetic mean