This remarkable and easy to understand answer can be found at the excellent website commonsenseatheism.com
Is Atheism a world view? (April 30,2009)
Clearly, atheism is not a religion, but there has been much talk in the comments about whether or not atheism is a "worldview." So, let's check the definitions (from the OED) of "atheism" and of "worldview" and see if one might be a species of the other.
atheism:disbelief in the existence of a god or gods
1. a particular philosophy of life or conception of the world2. a collection of beliefs about life and the universe held by an individual or a group
Given this definition, I do not see how atheism can be a worldview. I have compared atheism to a-unicornism: disbelief in the existence of unicorns. How is a-unicornism a "worldview"? It's not. Atheism and a-unicornism are each a single belief about one thing. Neither of these positions tell you anything else about the person who holds them: their morals values, their political views, their driving purpose, their explanations for life or the universe, their beliefs about magic or ghosts or elves, their rationality or their intelligence.
But, Bobmo wrote:
In other words, if there is no God, then x must be true (e.g. matter is eternal, or a multiverse exists; there is no absolute morality, etc.) The same cannot be said for A-unicornism, since the non-existence of unicorns carries no serious implications. I deny that atheism has such implications.
None of Bobmo's examples follow from the non-existence of gods. They may be true, but they are not entailed by atheism. As toweltowel replied: "Supposing that theism implies p, and that atheism is the denial of theism, it obviously does not follow that atheism implies [not-p]."
Neither an atheist nor an a-unicornist must believe in eternal matter, a multiverse, or moral relativism. And in fact, I'd bet millions of them don't.
Adiel Corchado has another try:
The difference between atheism and [a-unicornism] is that unicorns provide no answers to why the world exists, why we exist, whether morality is objective or subjective, what happens after you die, etc. If unicorns don’t exist... that changes nothing. If unicorns do exist that changes nothing. God’s existence or non-existence on the other hand changes everything.
I have never seen a definition of "worldview" that uses Adiel's criteria for something being a worldview. Both bare atheism and bare theism have no answers to why the world exists, why the world exists, whether morality is objective or subjective, or what happens after you die. For you to start answering those questions you have to adopt a worldview, like a particular brand of worldview naturalism or Christianity or extropianism.
Yes, even "theism" - in the bare sense that is the opposite of atheism - is not a worldview. Like atheism, theism is a single belief about one thing: the existence of a god or gods.
What else is entailed by belief in a god or gods? Absolute morality? The origins of life or the universe? The afterlife? The purpose of life? None of these things are entailed by theism, not even the origins of the universe. Not all gods are thought to be eternal, or creative. And not all theistic religions think that the gods can explain the origins of the universe, for example many varieties of Buddhism.
Atheism is the mere opposite of theism, and neither of these entail a long list of beliefs like a worldview does