The mathematical symbol for "average" is an italicized "x" with a horizontal line over it. The most common type of average is the mean, though other types exist.
"Mean" and "median" are both types of averages. "Mean" is when a group of numbers are added together, and then the group is divided by the number of items in the original group. The average of 13, 15 and 23 is 17 because the sum of the three numbers is 51. That number divided by 3, the number of items in the group, is 17. "Median," on the other hand, is the middle number in a group of consecutive numbers. The median of the list above is 15.