Duration

investopedia says:

A measure of the sensitivity of the price (the value of principal) of a fixed-income investment to a change in interest rates. Duration is expressed as a number of years. Rising interest rates mean falling bond prices, while declining interest rates mean rising bond prices. The bigger the duration number, the greater the interest-rate risk or reward for bond prices.

The duration number is a complicated calculation involving present value, yield, coupon, final maturity and call features. Fortunately for investors, this indicator is a standard data point provided in the presentation of comprehensive bond and bond mutual fund information.

It is a common misconception among non-professional investors that bonds and bond funds are risk free. They are not. Investors need to be aware of two main risks that can affect a bond's investment value: credit risk (default) and interest rate risk (rate fluctuations). The duration indicator addresses the latter issue. Short-term, intermediate-term and long-term bond funds will have different durations. For example, Vanguard's short-, intermediate- and long-term bond index funds generally have durations of around three years, six years and 11 years, respectively

Links

 * investopedia:Macaulay Duration
 * investopedia:Modified Duration