There are two components to what is typically called 'atheism':
The first is a knowledge claim. That is, a claim whether to have knowledge in a particular domain. In this case, the existence of a deity or deities. In general an atheist will have an 'agnostic' claim. This is a soft-negative claim that translates to "I have no knowledge" in regards to the existence of a deity or deities. It does not deny the existence of, or claim to have knowledge of, the lack of existence of a deity or deities. The opposite claim is a 'gnostic' claim. This is a claim to have knowledge regarding the existence or non-existence of a deity or deities. A person of faith or a religious believer in a deity or deities generally makes a 'gnostic' claim, although there can exist 'agnostic' theists as well. The person of faith or religious believer claims to 'know' a deity or deities exist.
The second is a belief claim. An atheist makes a no-belief claim. That is they say "I do not believe in a deity or deities". This comes from the term atheist or atheism itself: 'a', or without, 'theism' or belief in deities. A 'theist' in contrast makes a belief claim in one or more deities. A 'monotheist' claims belief in, or the primacy of, one deity. A 'pantheist' claims a belief in multiple deities.
When asked "do you believe in God?" an athiest will answer in the negative. However, if asked "do you believe there is no God?" an athiest will typically answer "I don't know" or in the negative as well. This is confusing for theists sometimes because of how thier own belief functions. To a thiest the opposite of a belief in a god seems it must be the belief in the lack of a god. This is to ignore the 'null hypothesis', or 'default state'. The null hypothesis is no belief, either way, in the existence of a diety or deities. Default state also refers to the fact that humans, until they are taught to believe or disbelieve, are natural atheists - they are not born with belief. This is the typical atheist position and is sometimes termed 'agnostic-atheist', 'agnostic-atheism', or even 'soft-atheism'.
Now there does exist a group of so-called 'hard-atheists'. This would be an athiest who believes there is no deity or deities in existence. This is an active belief claim. This would be a 'gnostic-atheist'. This is not what is typically meant by the general term atheism. Most agnostic-atheists would reject this claim on the grounds of lack of evidence. It is a claim easily dismissed, as well. Since this cannot be proven or not proven, the agnostic-atheist would say one must default to the null hypothesis - lack of belief - rather than an active disbelief.
These views, along with what is typically termed 'agnostic', usually meaing one who does not 'know' but has an active belief, can be summed up in the following table:
| Yes | No | |
|---|---|---|
| Theist | ||
| Agnostic | ||
| (Soft) Atheist | ||
| (Hard) Atheist |
| Yes | No | |
|---|---|---|
| Theist | ||
| Agnostic | ||
| (Soft) Atheist | ||
| (Hard) Atheist |
Note there is some overlap in what is typically called agnostic or atheistic. This is due the the shared professed lack of knowledge or gnosis, while the degree of uncertainty or lack of belief will typically cause a person to choose either agnostic or atheist as a label or identity. Many people who think of themselves as agnostics are more accurately atheists, they just have a misunderstanding of what the term means and implies. This is often due to conflating the hard-athiest postion with atheism in general.
To conclude, what is often thought of as atheism is actually the hard-atheist position. This is in actually a minority postion held by few self-identified atheists and is an inaccurate description of what they believe or don't believe.