A person that does not believe in the idea of a God either due to the lack of evidence supporting the idea, the massive trove of contradictions or both.
Not all atheists hate religion or support evolution, this is merely a claim from religious people unable to realise that their religion is a belief floating on faith, not a fact backed up with other facts.
Some reasons Atheists may use for no believing in religion
1- "You claim the Earth cannot have formed itself yet you say that it was made by an invisible sky man that formed themself?"
2- "If you read your bible you will see how cruel your god is and all the atrocities done in their name. Even if your god was real I wouldn't believe in someone like that."
3- "I decide to question things to get my knowledge rather than assume something so complex for me to understand instantly needs a creator. If fundamentalists actually listened to us and researched the theory of evolution (and making out
the fact of evolution doesn't exist either)then maybe they would actually realise that it makes more sense than their religion full of flaws and contradictions