The "average" or "mean" is a measure of central tendency. It's one of 3 measures of central tendency, and is the most popular one.
The "average" of a number of quanties, such as incomes, is usually taken to refer to their sum, divided by the number of incomes that have been summed.
In other words, the average of 5 numbers, N1, N2, N3 and N4 + N5 is:
(N + N2 + N3 + N4 + N5)/5.
That average is also called the "mean" of those numbers.
Another popular measure of centreal tendency is the "median".