Someone that thinks of a god or gods as impossible, or just doesn't really care. Atheism is a lack of belief, not a belief so atheism is not a religion. Atheists may or may not have a love of science. Some atheists have plausible reasons for being an atheist, while others don't. You can be one of the following kinds of atheists:
1) Someone that has had something personal happen with their religion and doesn't believe in god because of that personal problem
2) Angry atheist: Someone intolerant to religion.
3) retaliating atheist: Someone that fights back when theists (mostly Christians) pick a fight. NOTE: Some theists confuse this one with #2 because atheists don't want Christianity to be in the government or anything symbolizing the government. These include flags, post offices, schools, town/village/city halls, etc.
4) Someone that really doesn't care and so by their lack of belief, are classified as an atheist.
5) Agnostic atheist. This one is commonly confused as agnostics. There are no such things as agnostics in the belief system. You can be an agnostic atheist or an agnostic theist, but you can't be just an agnostic.
"I'm an atheist"
"I have had personal troubles with religion"
"I am intolerant to religion"
"Because it is the root of all evil"
"I'm an atheist and I don't think religion should be in the government"
"You are an angry atheist"
"No, I just don't want the government forcing religion on me."
"Are you a christian?"
"I don't really care"
"Are you an atheist?"
"Again, I don't care"
"I'm an agnostic atheist. I don't believe there is enough evidence to prove the existence of a god."