Compare the definitions below (from the American Heritage Dictionary, via the GUNTRUST.ORG Toolbar you can download FREE for your browser). Looking to the primary definitions, the dictionistic dictators of how we speak seem to treat Islam differently, in depicting Islamism as primarily referring to a mere faction within the body of Islam.
The faith, doctrine, system, and practice of a Catholic church, especially the Roman Catholic Church.
1. Adherence to the religion and beliefs of a Protestant church.
2. The religion and religious beliefs fostered by the Protestant movement.
3. Protestants considered as a group.
Is·lam·ism (s-lämzm, z-, sl-, z-)
1. An Islamic revivalist movement, often characterized by moral conservatism, literalism, and the attempt to implement Islamic values in all spheres of life.
2. The religious faith, principles, or cause of Islam.
Here is their definition of "-ism" itself:
1. Action; process; practice: terrorism.
2. Characteristic behavior or quality: heroism.
a. State; condition; quality: pauperism.
b. State or condition resulting from an excess of something specified: strychninism.
4. Distinctive or characteristic trait: Latinism.
a. Doctrine; theory; system of principles: pacifism.
b. An attitude of prejudice against a given group: racism.
More religionisms listed below–don't see any that take the "-ism" as referring to a mere faction of the root word it qualifies:
A diverse body of religion, philosophy, and cultural practice native to and predominant in India, characterized by a belief in reincarnation and a supreme being of many forms and natures, by the view that opposing theories are aspects of one eternal truth, and by a desire for liberation from earthly evils.
Bud·dhism (bdzm, bdz-)
1. The teaching of Buddha that life is permeated with suffering caused by desire, that suffering ceases when desire ceases, and that enlightenment obtained through right conduct, wisdom, and meditation releases one from desire, suffering, and rebirth.
2. The religion represented by the many groups, especially numerous in Asia, that profess varying forms of this doctrine and that venerate Buddha.
The religious system founded by Zoroaster and set forth in the Avesta, teaching the worship of Ahura Mazda in the context of a universal struggle between the forces of light and of darkness.