A person who believes in no God. This is also referred as
strong (positive) atheism in contrary to weak (negative) atheism.
A person of this belief is probably an Atheistic existentialist or Nihilist. Rejects a god and any transcendent will that should in some way dictate how humans should live. It rejects the notion that there is any "created" meaning of life and the world. The difference between them is that an Atheistic existentialist believes that individuals can create meaning in life themselves.
"So you don't believe in God?"
"No I don't"
"You are an Atheist?"
"A Hardcore Atheist"
"I don't believe in god or any transcendent will, at all."