In ordinary (non-technical) English the word average can be used for various concepts relating to a typical value of a list of numbers:
maybe the "most common" income, middle income in @Dave's line-up, or the number you get when you add all the incomes and divide by the number of incomes. If two people
disagree about "the average," they may need to discuss what each of them really has in mind.
In statistical terminology the first of these is called the mode (most common), the second is called the median (middle entry in a sorted list), and the third is called the (arithmetic) mean. [There are also 'geometric' and 'harmonic' means, but let's leave them for later.]
Sometimes mode, median and mean are all the same number (or nearly the same number).
If we have a list of heights of 25 randomly chosen college students (measured to
the nearest inch), we might get:
57 61 62 64 64 65 66 66 66 66 66 66 68
68 68 69 70 70 71 71 71 71 71 71 72
For this sample: the mode is 66, the median is 68, and the mean is 67.2.
However, for the data: 1, 2, 3, 3, 3, 4, 6, 7, 10, 15, 23, the mode is 3, the median is 4 and the mean is 77/11 = 7.
A billing agency might have 300 clerks making \$40,000 a year, 10 supervisors making \$80,000, and a CEO making \$800,000. Then the mode and median are both \$40,000, and the mean is \$43,730. The mode and median may seem more typical, but only the mean is directly related to the total annual payroll.
The harmonic mean is the reciprocal of the arithmetic mean of reciprocals of the numbers. It can be useful for finding average MPG (miles per gallon) for driving a car.
I live on a hill. If I drive a mile down the hill at 30 MPG, a mile on flat roads at 25 MPG and back home for a mile up hill at 20 MPG, my average gas mileage is not 25 MPG.
I use 1/30 gal. downhill, 1/25 gal. on flat roads, and 1/20 gal. uphill, for
a total of 0.1233 gal. or an average of 0.1233/3 = 0.0411 gal./mi.
So my average MPG is 1/0.0411 = 24.3 MPG. [Where liters and kilometers are used, a
customary measure of fuel efficiency is the number of liters to go 100km, and the arithmetic mean works fine for that.]
The geometric mean is used, among other things, to compute average interest rates or stock portfolio
returns. I will give links for that: Investopia and Wikipedia.