“ism” inherently means ideaology. a = not, theo= god, ism = ideaology.There is no "ideology" of atheism.
a distinctive practice, system, or philosophy, typically a political ideology or an artistic movement.
This is when you say the dictionary is wrong.
Atheism is the lack of belief in the existence of gods, and that's all it is. Any ideology (except a god-based one) may be attached to it.
Since there is no ideology of atheism, it doesn't have a "definition of good or bad". Atheism says nothing at all about good or bad; atheists may hold that morality doesn't exist, exists but is relative, exists but is objective or any other view they care to.
Finally, the definition of good or bad in use has nothing to do with nihilism. Specifically, holding that good and bad is relative does not make one a nihilist.