Atheism is the lack of a belief in the existence of one or more gods. An atheist is someone who does not believe that any gods exist.
It’s important to understand that this is different from explicitly believing that no gods exist. Consider the two statements below.
‘I do not believe that god exists.’
‘I believe that god does not exist.’
If a person gives the first of these two statements, they are saying that they are not convinced that god exists, but they are not saying anything about whether they are convinced that god doesn’t exist either.
If a person gives the second of these two statements, they are saying that they are convinced that god does not exist.
Both of these positions are atheist positions, but the first is known as agnostic atheism (or negative atheism) and the second is known as gnostic atheism (or positive atheism).
By far the majority of atheists are agnostic atheists. This means that they are not convinced that a god does exist, and they are not convinced that a god doesn’t exist either.