An exchange of cash flows in which one counterparty makes payments based on a fixed interest rate, the other counterparty makes payments based on a floating interest rate, and the counterparty paying the floating interest rate (and receiving the fix rate) has the right to end the swap before it matures. An investor might choose a putable swap if interest rates are expected to change in a way that would adversely affect the floating rate payer.
The additional features of a putable swap make it more expensive than a plain vanilla interest rate swap - the floating rate payer will pay a higher interest rate and possibly an early termination fee. The opposite of a putable swap is a callable swap, which allows the fixed interest rate payer to end the swap early.