Real Dictionary

-ism
suffix: -ism is used to form uncount nouns that refer to political or religious movements and beliefs.

-ism in American English
the act, practice, or result of
suffix: a suffix appearing in loanwords from Greek, where it was used to form action nouns from verbs (baptism). On this model, -ism is used as a productive suffix in the formation of nouns denoting action or practice, state or condition, principles, doctrines, a

-ism in British English
suffix forming nouns: indicating an action, process, or result

interstellar medium in British English
noun: the matter occurring between the stars of our Galaxy, largely in the spiral arms, and consisting mainly of huge clouds of ionized, neutral, or molecular hydrogen

ism in American English
a doctrine, theory, system, etc., esp. one whose name ends in -ism
noun: a distinctive doctrine, theory, system, or practice

ism in British English
abbreviation for: interstellar medium
noun: an unspecified doctrine, system, or practice