According to Me

An atheist is a person who does not believe in the existence of a god or gods. That’s it!

According to Miriam Webster

1. a disbelief in the existence of a deity

2.  the doctrine that there is no deity

According to Oxford

Disbelief or lack of belief in the existence of God or gods.

According to Wikipedia

Atheism is, in the broadest sense, the absence of belief in the existence of deities. Less broadly, atheism is the rejection of belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities.

Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists.

Origin and Etymology of atheism

Middle French athéisme, from athée atheist, from Greek atheos godless, from a- + theosgod