For the record, this list of definitions is pulled directly from the amazing resource that is the FAQ from r/atheism. I helped write much of that document and I feel no shame at stealing from it at need.

What is atheism?

Atheism, from the Greek ἄθεος (atheos), is defined as “The lack of belief in the existence of any deities.” In modern context, atheism can represent several different viewpoints, but is most commonly conceived of as a rejection of belief in gods.

A person can be both atheist and religious, provided that he or she believes in a religion that does not have any deities, such as some forms of Buddhism.

The word “atheism” is not a proper noun, so there is no need to capitalize it except in grammatically appropriate circumstances, such as the beginning of a sentence.

Atheists do not worship the All Powerful Atheismo nor the Goddess Athe, so any wordplay in that sense is invalid. How does it differ from theism?

Theism is a belief in at least one god. Thus, religions such as Christianity, Hinduism, Islam, Judaism, and Zoroastrianism are all considered theistic.

Any person who believes in one or more gods is a theist.

Any person who is not a theist is an atheist.

Atheism is not a religion. It has no dogma, no credo, no congregation, no holy leaders. It’s just not believing gods are real.

What is agnosticism?

An agnostic is someone who claims they don’t know (“weak agnosticism”) or it is not possible to know (“strong agnosticism”) for certain whether or not gods exist. The term agnosticism comes from Greek: a (without) + gnosis (knowledge).

What is the difference between agnosticism and atheism?

Atheism and agnosticism are not mutually exclusive. “Agnosticism” is not some third position which is neither “atheism” nor “theism”. They are different answers to different questions, in this case “Do you believe that any gods exist?” and “Do you believe it is possible to know whether any gods exist?”.

Anyone who does not hold a belief in one or more gods is an atheist. Someone who holds an active belief in the nonexistence of particular gods is specifically known as a “strong” or “explicit” atheist, as opposed to “weak” or “implicit” atheists who make no claims either way.

On the other hand, the vast majority of atheists are at least technically agnostic, even if they are willing to treat fairy tales about Zeus or Allah with the same contempt that they treat tales about unicorns and leprechauns. Describing yourself as “Just an agnostic”, or stating “I’m not an atheist, I’m an agnostic” makes about as much sense as saying “I’m not Spanish, I’m male.”

What are anti-theism, state atheism, and secularism?

Anti-theism - The active or inactive attempts to put an end to theism, often as a reaction to anti-scientific thought, bigotry, and questionable morals propagated by many theists. Many self-identified vocal atheists are in fact anti-theists on some level. There are a few reasons for this, perhaps most significantly the fact that anti-theists tend to have a lot more to say. Make no mistake: there are a lot of atheists who are not openly hostile to religion. The squeaky wheel, however, tends to get the grease, and atheists who have no major gripes with religious belief have less motivation to speak out. The most public example of an atheist who is not an anti-theist is S.E. Cupp (though many are skeptical of her atheism). One of the better examples of an anti-theist is Christopher Hitchens.

State atheism - This is the “theocratic” form of atheism. It is the ideology that atheism should be enforced by the government, as it has been under many communist governments. Most members of r/atheism are secularists and oppose this notion. Under current and historic Communist governments, the primary desire of the state is economic and social reconfiguration of society, and religious attitudes are an outgrowth of those larger objectives.

Secularism - Secularism in typical usage refers to the government not respecting any religion or religious beliefs. In this way it does not promote any form of theism or atheism. An example being that government recognized “Day of Prayer” is not secular, nor would a government recognized “Day of Disbelief in Deities” be.

In the context of organizations like the Secular Student Alliance, “secular” also denotes an absence of religious affiliation or purpose. Anything that is not specifically religious is secular.

What is Deism?

Deists believe that a higher power created the universe long ago but is not or no longer actively present in the world and does not intervene in its affairs. Because of their belief in this “higher power,” often thought of as a god, they don’t qualify for the “atheist” label. Some definitions consider Deism to a subset of theism, while others consider the two terms to be distinct- there are occasional minor squabbles about this, but the consensus in /r/atheism seems to be that the former is correct.

However, for practical purposes there is little difference between a deist and an atheist: Most deists do not engage in the usual religious practices of praying, worshiping, rituals, restrictions in diet and/or lifestyle or regarding a central holy doctrine. Deists share the atheist position that there is no deity active today.

Because of this similarity, some atheists will claim deism when asked about their religion. Deism places no practical obligations on its adherents, yet does not bear the heavy public stigma associated with atheism. Americans, for example, can benefit from the respect afforded many well-known Deists among the nation’s fathers: Benjamin Franklin, Thomas Jefferson, Thomas Paine, James Madison, George Washington. Because the deist God takes no action in the present universe, its existence is effectively meaningless because no special actions are the result of its existence. That is to say, there’s effectively no difference between a godless universe and one with a Deistic god, so asserting one’s existence is like asserting the existence of an Invisible Dragon in your Garage.

What is Igtheism/Ignosticism/Theological Non-cognitivism?

Do you believe that frajingle majibbity jibbity moop? Why or why not?

If your answer is “How am I supposed to believe in that? It’s a nonsense statement!”, you’re already half way to understanding Ignosticism and Theological Noncognitivism, because that’s how they feel about the statement “God exists”.

It’s an acknowledgement of the fact that the word “God” has meant billions of different things to billions of different people, from “a magical man who lives on top of that mountain and demands that we sacrifice goats to him” to “a mystical love-force that mystically touches people’s hearts through completely undetectable means” to “like, the entire Universe is God, man. Whoah!”, and that many of these definitions are themselves silly, unfalsifiable, self-contradictory, or incoherent. Indeed, when backed into a corner, apologists like playing silly games with the definitions of words like “exist”, too.

Therefore, ignostics and theological non-cognitivists hold that it’s futile to try to make statements about “God” unless the person you’re talking to is willing to rigorously define what they mean by “God” first, preferably using falsifiable statements that constrain anticipation, rather than nonsense about “faith”. The difference between the two is that ignosticism states that other positions assume too much given the lack of a definition while theological non-cognitivism rejects the idea that the term “God” is cognitively meaningful. Due to the similarities between the two (most people who hold one stance also hold the other) the terms are often treated as synonymous in casual conversation.

See also: Newton’s Flaming Laser Sword, a stronger version of Occam’s Razor which states that statements that don’t actually mean anything are useless.

What are Secular Humanism, Transhumanism, Skepticism, Rationalism, Empiricism, Positivism, Postmodernism, Materialism, Naturalism, Nihilism, etc. ?

These are all terms for philosophies and worldviews which are compatible with and often associated with atheism. Indeed, they are often confused for atheism by people who don’t realize that “atheism” means nothing more and nothing less than a lack of belief in any god or gods. Being an atheist doesn’t necessarily make you any of these:

Secular Humanism is a philosophy that “embraces human reason, ethics, social justice and philosophical naturalism, whilst specifically rejecting religious dogma, supernaturalism, pseudoscience or superstition as the basis of morality and decision making.” Basically, it’s about being good for goodness’ sake, rather than because some invisible bully in the sky told you to.

Transhumanism is a philosophy which states that the human condition can and should be improved, and improved drastically. It advocates the use of science and technology to fight disease, hunger, poverty, and aging, to enhance the physical and mental capabilities of individuals, and eventually to colonize the solar system and beyond. Prominent Transhumanist Eliezer Yudkowsky explains that, in his opinion, Transhumanism is simply Humanism simplified, with no “you should stop helping people after this point” line. Critics of Transhumanism tend to poke fun at its Science-Fiction trappings and its overly optimistic attitude.

Skepticism is, basically, the philosophical position that all beliefs should be supported by evidence. Beloved skeptic Carl Sagan coined the phrase “Extraordinary claims require extraordinary evidence”. It is far easier to convince a skeptic that you have a pet cat than that you have a pet blue whale, and far easier to convince them that you have a pet whale than that you have a pet unicorn. Applied Skepticism is often known as The Fine Art of Baloney Detection.

Rationalism is the belief that Logic is supreme. Logic is extremely useful, but limited by the fact that one’s conclusions are only as reliable as the premises one feeds into the logical framework. Proponents of the strong form of Rationalism (such as René Descartes) love making a priori arguments and tend to fall prey to the “Garbage In, Garbage Out” effect. When you apply sufficient skepticism to your choice of premises, however, Rationalism begins to resemble Rationality, which is a potent tool indeed.

Materialism is, essentially, the position that there’s no such thing as magic, and that everything that exists is made of either matter or energy. It doesn’t rule out the existence of types of matter or energy with which we are unfamiliar, but tends to frown on pseudoscientific misuse of the word “energy” to mean “magical woo-woo”. Methodological Naturalism, an important component of the scientific method, basically says “In the absence of any evidence to the contrary, let’s at least do science as though materialism is true. Because otherwise what’s to stop us from assuming that everything is run by invisible undetectable pixies?”

Empiricism (Particularly Pragmatism) is the philosophical position that, basically, observation of the real world is the only way to learn things about the real world, and that endeavors such as “faith” and “divine revelation” are useless for gaining knowledge. If you want to make an accurate map of a city, you can’t sit in your room being “inspired” to draw random lines on a piece of paper, you actually have to observe the layout of the city in some way. Empiricists popularized and began to formalize the process of using experiments as a method of asking the universe questions.

Positivism began, basically, as the statement that “that which cannot be settled by experiment is not worth thinking about”. When it quickly became obvious that the philosophical validity of that statement itself could not really be settled by experiment (d’oh!), gentlemen like Karl Popper got a hold of it and molded it into a refinement of philosophical empiricism and methodological naturalism.

Postmodernism is basically a reaction of artists and squishy-philosophers who objected to the idea of scientists and more rigorous philosophers harshing their mellow. When taken to its extreme, it states that all reality is just made up of social constructs, and that facts are just, like, your opinion, man.

Nihilism is a collection of philosophies which state that such-and-such a thing is without inherent meaning. Existential Nihilists in particular argue that life and existence itself are without inherent meaning, while Moral Nihilists argue that any established moral values are just social constructs. That’s not to say that being a nihilist necessarily makes one a psychopath or a depressed teenager- most nihilists are perfectly capable of making their own meaning to things, they simply reject the idea of there being any deeper meaning than that.

You’re not using the correct definition of a word!

You’re welcome to disagree, but these are the definitions that I’m presenting to the community and they reflect the original meanings of the words and the views of a lot of atheists around the world. Before engaging in a debate that hinges on these terms, it may be necessary to come to an agreement on what those terms mean. Remember, it makes a lot more sense to ask someone what their position is than it does to dictate their position to them based on your understanding of terminology.