Defining the term: Atheist.
In general discussions broad definitions are acceptable. Such conversations are usually between people who have some sense of connectivity and understanding and thus know where each other are coming from. So the focus is more on brevity rather than clarity.
But then there is the matter of debates and other such more finessed discussion. In such forums clarity becomes increasingly vital. The meaning of what you say, while clear to you, needs to be as clear to who you are talking to. So defining terms and semantic issues is something that we should clear up as best possible.
So what is an atheist? The common presumption is that an atheist is someone who denies the existence of God. That may suffice for a casual conversation but logically speaking it runs into some issues depending on the individuals involved. So lets break the word down.
Lets start with the word Theist. From the ancient Greek word theos which means god. A theist simply put is someone that believes in god or gods. Pretty straight forward. This does not tell us which god(s) the individual believes in or what reasons they have for holding such beliefs. It only tells us that they believe in some form of god(s).
Next we come to the prefix ‘a’ meaning ‘not’ or ‘without’. Someone who is amoral would be someone without morals. Something that is atypical would be not typical. Thus at its simplest meaning an atheist is someone who is not a theist or who is without a belief in god(s). Again we don’t know why this individual does not believe in god(s). They could be part of a religion that does not include gods or they could simply find no reason to believe in gods. There are a multitude of reasons for not believing in gods and which of them the individual holds to is not defined by the simple word atheist.
While we are on the subject we may as well cover the word that many turn to thinking it to be a third option. Agnostic. A lot of people take the word agnostic to be a sort of middle ground between theist and atheist. The trouble is when we get to the clear definition of the terms we find its not quite so.
The term agnostic was famously coined by Thomas Huxley. Huxley used the word to define his philosophical position that the existence of God was not something that could be known. Many people take this term to mean that by not claiming to know if god(s) exist they avoid taking sides in the often fierce conflict between theists and atheists.
Huxley based the word on the (again) ancient Greek word gnosis meaning knowledge. A gnostic(a term many people forget about) is a person that claims to know something. Typically used in relation to god(s). The Gnostic sect of Christianity for example claimed to have special knowledge about God. The prefix ‘a’ again is applied to the word to give us agnostic which means someone that is not a gnostic or is without special knowledge about god(s).
The thing is that theist/atheist and gnostic/agnostic are not mutually exclusive terms. That is theist/atheist refer to matters of belief (which don’t require direct knowledge) and gnostic/agnostic refer to matters of knowledge(which influence the nature and depth of one’s beliefs). So you can be one of both categories. Atheist and Agnostic. Theist and Agnostic. Theist and Gnostic. Atheist and Gnostic.
It is important to note that by the nature of their meaning theist/atheist and gnostic/agnostic are binary words. That is if you are not one then you are by definition the other. They are either/or. A person who is not a theist is an atheist. If you do not have an active belief in god(s) then you are an atheist. It does not matter if you do not claim to know if there is a god(s) or not. It is the simple lack of belief in god which causes the word atheist to become applicable.
So long story short. For whatever reason if a person does not believe in gods at a given moment they are an atheist. Its not a commitment. They may be a theist some short time after that and shift back to atheist repeatedly. But at any given moment what a person is, is based on what they do or do not believe at that moment.
There are a number of various types of atheists. The sort of atheist that most think of when they hear the term (someone that actively believes there is no god(s) or disbelieves in god(s)) can be referred to by a number of phrases. The most common phrase is Strong Atheist. Richard Dawkins refers to this sort of atheist as an Explicit Atheist. The individual may consider themselves to know there is no god(s) and thus believe themselves to be a Gnostic Atheist.
Those who do not claim to know there is no god(s) are commonly called Weak Atheists. This does not mean they are weak in their philosophical understanding. As such this term is often frowned upon by such atheists and thus they usually refers to themselves simply as an atheist as its simplest and basic definition fits them sufficiently. Dawkins uses another term. He refers to such atheists as Implicit Atheists. And of course as such atheists do not claim to know if there is a god(s) they may self identify as Agnostic Atheists.
So why is there so much consternation about these terms (trust me there is quite a bit of hullabaloo)? As society spread our awareness of other cultures grew with it. As we learned of other cultures our world views changed. But before this happened most people lived within monolithic communities with singular beliefs distributed within the community. Someone that did not accept such a belief could be said to reject the particular god in question. But as the world became more interconnected more concepts of gods were introduced. So it became increasingly difficult to become aware enough of all the claims of god(s) being proffered. In such a case despite rejecting specific claims for god(s) an honest philosopher would have to admit to not being able to contend with all the possible/available claims for god(s) and thus not be able to claim an absolute rejection. Thus they fall to a simple agnostic atheist position. Neither believing in gods nor claiming to know there are no gods.
But there is a further reason that there is so much insistence that atheists reject God and it has to do with the Christian concept of sin (or at least some Christians). For many Christians sin is a deliberate and conscious act of turning away from God. This fits into their sense of justice as someone that commits a wrong without knowing they do so does not harbor deliberate evil in their hearts. It causes their mind to balk at the idea that a person who is good in their heart could be condemned to eternal damnation. So sin must be a deliberate act in their mind. Thus an atheist must be deliberately turning away from God.
This implies, in their opinion, that the atheist knows God exists but chooses to turn away from him and deny his existence. This is why the phrasing is so important to Christians when confronting nonbelievers. The idea of an implicit atheist merely not believing instead of directly rejecting God causes cognitive dissonance within their mindset regarding God, sin, and justice. So they frequently insist that an atheist is someone that denies, rejects, or refuses God.
There is a further reason for theists to want to define atheists as claiming god(s) does not exist. The nature of logic dictates that whoever makes a positive claim about something has to back that claim up. If someone claims that god(s) exist then they have to provide the evidence and reasoning supporting their claim. Then the skeptics can examine the evidence and reasoning to see if it holds up. As this proves to be very difficult for theists they attempt to deflect their burden of evidence by trying to reverse it and pin it upon the atheist.
To this end they attempt to posit the atheist’s position as making a positive claim that god(s) does not exist. If posited as such it becomes the burden of the atheist to show evidence and reasoning that god(s) does not exist. But as the atheists position is by nature a reactive one it cannot argue for its case without a claim for god(s) being presented. The typical complaint is you cannot prove a negative. But the simple fact is that without a claim for a defined god(s) the atheist has nothing to argue against. This tactic is simply the theist trying to build a strawman of the atheist’s position (unless the atheist actually makes the positive claim for some reason). It is a dodge. It is spin. It is intellectually dishonest.
Atheist: A person lacking a belief in god(s) for any reason (or for no reason at all). Their lack of belief in god(s) may or may not be due to a particular religious belief. But on its own it is nothing more than a label for the fact that the individual does nto at the moment believe in god(s).