A piece or chunk of a large swap (one with a large notional amount) that is transacted with one swap house. In other words, the transaction may be done in several different chunks at several different points in time with a number of swap houses. For example, a $300 million transaction might be broken into five pieces (tranches) each of which may or may not have the same notional amount, but the sum of the notional amounts for all five tranches would be equal to $300 million. Typically, the transaction would have the same maturity (swap tenor).