Average (Mean)

A measure of central tendency calculated by adding the grand total for a set of continuous (a.k.a. variable or numerical) values and dividing it by the total count of those continuous values. Also known as the Mean, it is often a reliable measurement when those continuous values being measured have a normal distribution.  Otherwise if it's a non-normal distribution that is positively or negatively skewed, then the Mean would no longer be a trustworthy representation of the central tendency for that distribution. In those cases, the Median would be more reliable. The average is represented by the Greek letter μ (pronounced "mu") for a population mean or X-bar for a sample mean.
Related StatStuff Video: 
   -Section 5-Measure Phase, Lesson #11 - Central Tendency