Wat is Constant-maturity swap?

A constant-maturity swap (CMS) exchanges a floating leg whose rate at each reset is the par swap rate of a fixed tenor (the “CMS rate”, for instance the ten-year swap rate observed at the reset) against a fixed or another floating leg. A CMS caplet pays $\max(S(T)-K,0)$ on the CMS rate observed at $T$, once, at a date $T_p$.