To give an entirely pedantic description of what Atheism is, we must first describe what constitutes a religion, as opposed to any other belief.
For instance: How exactly is fanatic liberalism separate from fanatic Judaism? What about Star Wars fanatics? What about those who are fanatics about "science," holding Richard Dawkins to a gold standard?
The cleanest answer I seem to be able to give is: Religions suppose the existence of things that are inherently, undivisibly minds.
Spirits, souls, gods, all these things have it in their base nature to be agents and minds and actors, distinct from rocks, plants, animals and machines: Humans form a grey area.
Modern science tells us that the human mind is a phenomena that arises from the complex interactions of billions of neurons in the brain. Nothing less, and more significantly, nothing more. Brain damage from injuries resulting in changes of personality is perhaps the clearest indicator that the mind in a deep sense is the brain.
The human mind is a fact of nature, it is a process that takes place in human brains, it is made of neural impulses and neurons and cells and proteins, etc.
The human mind is not fundamental, it is made of parts.
Most of science is founded on the belief that the only fundamental things in the universe is fundamental particles.
Religious beliefs distinguish themselves from the beliefs of Libertarians, "Science-ists," Star Wars fans, Communists, Football team fans and others, bu explicitly including fundamental minds, minds that cannot be divided into parts, be they spirits, gods or souls.
So an Atheist is a person who do not believe in the existence of fundamental minds and agents.