Compound Annual Growth Rate - CAGR

(source http://www.investopedia.com/terms/c/cagr.asp)

The year over year growth rate of an investment over a specified period of time.

Calculated by taking the nth root of the total percentage growth rate where n is the number of years in the period being considered.

This can be written as: (Current Value/Base Value)^(1/# of years) - 1.

CAGR doesn't represent reality. It's an imaginary number that describes the rate at which an investment grew as though it had grown at a steady rate.