One of the two legs of a tranche (vanilla tranche/ plain vanilla tranche), with the other one being a tranche default leg (tranche protection leg);

It represents the regular amounts that the protection buyer pays to the protection seller, against the credit protection. In return, the protection seller stands ready to compensate the protection buyer against the tranche losses arising under the credit structure.

In a related, but different context, every tranche carries a coupon (interest) as compensation for its holder (investor) commensurate to the risk profile of the tranche, among other factors. For riskier tranches in a securitized structure the tranche coupon is higher depending on the amount of risk involved (as defined by the overall riskiness of underlying collateral). The coupon consists of a reference rate (floating rate index) adjusted with a specific spread.

The tranche coupon leg is also known as a tranche premium leg.