The suffix -ism is a noun suffix. That is, when added to words or word roots, -ism forms nouns. It comes from the Greek noun suffix -ismos and means roughly “the act, state, or theory of.” Nouns that end in -ism often have related verbs that end in -ize (criticism/criticize), related agent nouns that end in -ist (optimism/optimist), and related adjectives that end in -istic (optimistic).
