Central Tendency

The term average is used all the time and is a way to summarise a group of values.
There are a number of different types of averages that are commonly used:
Arithmetic Mean - This is the average to most people and is the total divided by the number of items. this is the most widely used measure of central tendency. Any reference to 'mean' implicitely refers to the arithmetic mean.
Median - This is the middle of the distribution when sorted in to ascending order.
Mode - This is the most frequently occurring value in a distribution.
Weighted Average - Similar to arithmetic mean except that some of the values have different weightings.
Midrange - This is the average of the largest and smallest values.
Geometric Mean - This is the "nth root of the product of the distribution".
Harmonic Mean - This is used to average rates rather than simple values and is really only used in Engineering.