Planned Amortization Class (PAC) Tranche

What Does Planned Amortization Class (PAC) Tranche Mean?
A class of tranche in a planned amortization class (PAC) bond that receives a primary payment schedule. As long as the actual prepayment rate is between a designated range of prepayment speeds, the life of the PAC tranche will remain relatively stable. This tranche of the PAC bond receives some measure of protection against prepayment risk.

Investopedia explains Planned Amortization Class (PAC) Tranche
The measure of prepayment risk protection, which includes both contraction and extension risk, is limited by the size of the companion bond and the speed of prepayment. If the speed of repayment is too slow (below the lower PAC collar), the life of the PAC tranche is extended; if the speed of repayment is too fast (above of the upper PAC collar), the life of the PAC tranche is shortened.