A suffix indicating an act, a process, the result of an act or a process, a state; also, a characteristic (as a theory, doctrine, idiom, etc.); as, baptism, galvanism, organism, hypnotism, socialism, sensualism, Anglicism.
The English suffix -ism was first used to form a noun of action from a verb. For example, baptize (or literally derived from "to dip") becomes "baptism." It is taken from the Greek suffix -ismos, Latin -ismus, and French -isme, that likewise forms abstract nouns from verbal stems.