A Swaption is an Option on a Swap, and can be used to buy or sell the right to conclude a Swap over a certain period. The underlying Swap is stated at the outset and is defined by its Notional amount, Maturity and the fixed and floating rate that are used as reference rates.

(See Chapter 49 Managing working capital of the Vernimmen)

To know more about it, look at what we have already written on this subject