A bond option which gives the holder the right, without the obligation, to sell a specific bond at a given price on or before a preset expiration date. Typically, a bond put option will have a lower value if the long-run interest rate is lower than the spot risk-free rate and, likewise, if the rate of spot rate reverting to long-run rate level is low. In the same manner, the value of a bond put option will be higher if the long-run interest rate is higher than the spot rate and if the reversion rate is low.