What is an "ism"? As in Atheism, Mormonism, Darwinism, Zoroastrianism, etc. ?

Here is what the dictionary says;

-ism
?
a suffix appearing in loanwords from Greek, where it was used to form action nouns from verbs (baptism); on this model, used as a productive suffix in the formation of nouns denoting action or practice, state or condition, principles, doctrines, a usage or characteristic, devotion or adherence, etc. (criticism; barbarism; Darwinism; despotism; plagiarism; realism; witticism; intellectualism).

?noun
a distinctive doctrine, theory, system, or practice: This is the age of isms.

Sounds like atheism isn't "non-belief" at all. I sounds like a doctrine, theory, system, or practice.

Why do atheists think it only means belief? I don't see that word in any of the definitions.

"ism" is just a common designation for "belief".

you forgot racism Lil

A means of classification.

The suffix -ism denotes a distinctive system of beliefs, myth, doctrine or theory that guides a social movement, institution, class or group. ...

-ism?
a suffix appearing in loanwords from Greek, where it was used to form action nouns from verbs (baptism); on this model, used as a productive suffix in the formation of nouns denoting action or practice, state or condition, principles, doctrines, a usage or characteristic, devotion or adherence, etc. (criticism; barbarism; Darwinism; despotism; plagiarism; realism; witticism; intellectualism).

All we are saying, is give peace a chance......

Except atheism is derived from theism:

a-, an-

Greek: a prefix meaning: no, absence of, without, lack of, not

There you go. So atheism is NOT a religious belief, doctrine, etc.

More information provided in the link below.

When people talk about "isms," they are referring to some "distinctive doctrine, theory, system, or practice" like liberalism, communism, conservatism, or pacifism. Atheism has the suffix "ism," so it belongs in this group, right? Wrong: the suffix "ism" also means a "state, condition, attribute, or quality" like pauperism, astigmatism, heroism, anachronism, or metabolism. Is astigmatism a theory? Is metabolism a doctrine? Is anachronism a practice? Not every word that ends in "ism" is a system of beliefs or an "ism" in the way people usually mean it. Failure to realize this can be behind other errors here.