Islam is a religion, and like any other faith, it is internally diverse. Islamism, by contrast, is the desire to impose a single version of Islam on an entire society. Islamism is not Islam, but it is an offshoot of Islam. It is Muslim theocracy.

In much the same way, jihad is a traditional Muslim idea connoting struggle—sometimes a personal spiritual struggle, sometimes a struggle against an external enemy. Jihadism, however, is something else entirely: It is the doctrine of using force to spread Islamism.

President...