"Atheism is a lack of belief in gods, from the original Greek meaning of "without gods." That is it. There is nothing more to it. If someone wrote a book titled "Atheism Defined," it would only be one sentence long."
While this definition is accurate from the standpoint of semantics, it is worth noting that there is a more aggressive form of atheism, sometimes called "hard atheism", which is defined by the belief that God or gods do not exist. There is a significant difference between lacking belief and possessing belief in a negative state of affairs or "negative fact". In the former case, no belief whatsoever is present and atheism is simply a position of skepticism, sometimes called "soft atheism", which leaves open the possibility that God or gods exist or do not exist. In the latter case, the existence of God or gods is negated. We can then construe hard atheism as the antithesis of theism, which is the belief that God or gods do exist. Both positions are speculative, and both involve presence of belief. Consequently, soft atheism, or what we can consider the most common species of atheism as indicated by the American Atheists, and also other members of the New Atheism movement, such as Dawkins and Harris (who, as scientists, openly admit the possibility that God or gods exist), turns out to be the more open-ended and neutral positions of the three, with hard atheism and theism as bi-polar extremes.
However, any good analysis of a term should not be limited to just a definition. We must analyze the meanings of the terms that comprise the definition. What this means is that atheism and theism are literally meaningless until we 1) define what those terms mean (which we have done in an oversimplified fashion above), and 2) define the critical terms of those definitions. The critical terms, present in the definitions of both atheism and theism, are "God" and "gods".
For sake of argument and simplicity, let's then focus on the following two statements: "Jack believes that God exists" (theism) and "Jill does not believe in God" (soft atheism). What could Jack and Jill possibly mean by expressing these beliefs? It all depends on how they define "God". Suppose both Jack and Jill define "God" similarly as a personal and sentient being who is omnipotent, omniscient, omnipresent, and omnibeneficent, who exists independent of the universe, and who is the creator of the universe. In this case, based on the definitions of atheism and theism, Jack is most definitely a theist, and Jill is clearly an atheist. This seems fairly cut and dried.
Now suppose we introduce another set of characters:
- Donald defines "God" as the non-sentient universe.
- Daisy understands "God" as a sentient universe.
- Mickey defines "God" as the sum total of energy in the universe, which has sentience.
- Mini believes that "God" is the non-sentient forces of nature.
Based on the original definition of theism, all the above mentioned characters belief in God, yet clearly not the same god. Yet, if we are to assume that for each character, "God" is a proper noun that denotes a unique being or thing, Donald, Daisy, Mickey, and Mini are all clearly atheists according to Jack. Jack believes a particular type of being exists, which he calls "God". Based on his own understanding of that term, Donald, Daisy, Mickey, and Mini do not believe in that god. That is atheism, or lack of belief in God. By the same logic, Jack is an atheist, relative to the theism of Donald, Daisy, Mickey, and Mini. In fact, we could argue that all five characters are atheists, relative to the unique understanding that each has of the term "God".
What about Jill, the soft atheist? Jill does not believe in a personal and sentient being who is omnipotent, omniscient, omnipresent, and omnibeneficent, who exists independent of the universe, and who is the creator of the universe. However, Jill does belief that the universe, energy, and forces of nature exist. Hence, by definition, Jill is a theist relative to the beliefs of Donald, Daisy, Mickey, and Mini, but still an atheist relative to Donald's belief. This also sheds light on what some atheists suggest, which is that everyone, including the Christian, is an atheist relative to some god or gods. Christians do not believe that Zeus exists; therefore, they are atheists relative to that concept of god. One might even argue that Christians, Jews, Muslims, and Hindus are all atheists relative to each other, because each of their doctrines clearly define "God" in a unique way, in some cases with characteristics that contradict the other. They do not believe in the same and identical god, though it is convenient to say they do as a way of constructing a superficial and universal concept.
What this suggests is two fold: 1) atheism and theism, as concepts, are meaningless when ascribing them to a unique individual, unless we apply some definition of "God" or "gods". This definition could be from the person holding the belief or lack of belief, or the person who is doing the ascribing; and 2) most everyone is both an atheist and theist, relative to some definition of "God" or "gods". In short, one person's theism is another's atheism, and one person's atheism is another's theism.
What this also means is that we have to be careful not to pigeon hole people with the terms "atheism" and "theism" based simply on our own subjective understanding of those terms and the terms "God" and "gods". That someone is a theist or atheist may be either true or false, based simply on definition and semantics.