Someone who does not believe in a deity.
Does not need to be "tolerant" of other people's silly, unfounded ideas when he knows full well that people kill each other over said silly unfounded ideas.
"Intolerant" atheists are usually dismissed as being "close-minded" or spiteful, when in fact they merely actively debate against the idea of using faith as a means of acquiring truth.
They debate it not because they feel superior, but because they know that faith is a dangerous idea with significant consequences in the world.
Atheist:"I think Mike's belief in green fairies is kind of ridiculous"
Theist:"I agree, Mike will believe anything!"
Atheist:"I think your belief in God is kind of ridiculous as well"
Theist: "Hey, don't bash my beliefs man!"
There should be no taboo in debating religion, as there is none in science, politics, or any other forum