The term durable, as it pertains to retirement income, implies that you can’t outlive the income, and neither can the market take away, or erode the income.  Examples of durable income would include Social Security, or pension income.  For the income to be durable, the source must offer guarantees upon which the investor can rely for the production of that income.  In contrast, non durable income would simply be the amount that we hope to sustain from a non guaranteed source, over time.