Best defined by example. If you invest $100 today and make 5% in the first year and reinvest ($105) and make 8% in the second year, the compound annual growth rate is 6.489%. The calculation is $100x1.05x1.08=$113.4 which is what you end up with at the end of year two. The average return is (square root(113.4/100) -1)= 0.06489 or 6.489%. Note 1. If we had three compounding periods we would take the cubic root (power of 1/3). Note 2. If we had invested at exactly 6.489 in both periods, we get $100x1.06489x1.06489=$113.4. Note 3. The example is directed to a return - but CAGR could be applied to earnings growth, GDP growth, etc.