It’s in any dictionary, and this reflects the broadest or common usage, and as was explained this definition is 100% inclusive of all atheists, the narrower definition you’re peddling to troll with is not.
Atheism
noun
- disbelief or lack of belief in the existence of God or gods.
Wikipedia
*"Atheism, in the broadest sense, is an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities.
Nonbelievers contend that atheism is a more parsimonious position than theism and that everyone is born without beliefs in deities; therefore, they argue that the burden of proof lies not on the atheist to disprove the existence of gods but on the theist to provide a rationale for theism."*
Definition
“Atheism is commonly defined as the absence of belief that any deities exist. Implicit atheism is “the absence of theistic belief without a conscious rejection of it” and explicit atheism is the conscious rejection of belief. Positive atheism is the explicit affirmation that gods do not exist. Negative atheism includes all other forms of non-theism. According to this categorization, anyone who is not a theist is either a negative or a positive atheist.”