The "duration" of a bond is a measure that reflects the sensitivity of the bond price compared to changes in interest rates. It indicates the average weighted maturity (in years) of the bond's cash flows and provides insight into how the value of the bond responds to interest rate movements.

Duration is often used by investors to assess interest rate risk. A high duration means that the bond price will react strongly to interest rate changes, while a low duration means that the bond price is less sensitive to interest rate fluctuations. The rule of thumb is that if the duration is 5 (the average weighted maturity (in years)) and the interest rate rises by 1%, the value of the bond falls by 5%.

For the connoisseurs, there are two commonly used forms of duration:

1. Macaulay duration: This is the weighted average time (in years) required to recover the present value of all future cash flows from the bond. It is often used for theoretical calculations and is useful when comparing bonds with different maturities and coupons.
2. Modified duration: This is a modified version of the Macaulay duration that indicates how the bond price changes as a percentage for a 1% change in interest rates. It is more practical for estimating interest rate sensitivity in investment portfolios.

In essence, duration helps investors better understand and manage the interest rate risk of a bond.