To find the mean (or average) of a series of numbers, for example 1,2,3,4,5 & 6, add them all together for a total of 21; then divide by the number of numbers (6) to give the mean (or average) of 3.5.

In contrast, when half the data of a set are above a point and half below, that point is the median. The difference between mean and median can be quite significant, but one often sees the terms used wrongly even in technical contexts.