Atheist
a person who denies or disbelieves the existence of a supreme being or beings.
http://dictionary.reference.com/browse/atheist
^^^ An atheist can take one of two positions. Either to simply disbelieve in the existence god or deny the existence of god.
A convo with two atheists; one weak, one strong, may go something like this:
Q: Do you believe in god?
WA: No.
SA: No.
Q: Why don't you believe in god?
WA: There is no current evidence to support the idea.
SA: All of the evidence says the existence of a god is impossible.
^^^ One atheist disbelieves in god. The other denies the existence of god.
Here is how the Oxford English Dictionary defines "atheism":
atheism
Disbelief in, or denial of, the existence of a god.
disbelieve
1. trans. Not to believe or credit; to refuse credence to: a. a statement or (alleged) fact: To reject the truth or reality of.
deny
To contradict or gainsay (anything stated or alleged); to declare to be untrue or untenable, or not what it is stated to be.
Logic. The opposite of affirm; to assert the contradictory of (a proposition).
To refuse to admit the truth of (a doctrine or tenet); to reject as untrue or unfounded; the opposite of assert or maintain.
To refuse to recognize or acknowledge (a person or thing) as having a certain character or certain claims; to disown, disavow, repudiate, renounce.
Note that the OED definition covers the whole spectrum of atheist belief, from weak atheism (those who do not believe in or credit the existence of one or more gods) to strong atheism (those who assert the contrary position, that a god does not exist).
Here is Webster's definition of atheism:
atheism n 1 a: disbelief in the existence of God or any other deity b: the doctrine that there is neither god nor any other deity--compare AGNOSTICISM 2: godlessness esp. in conduct
disbelief n: the act of disbelieving : mental refusal to accept (as a statement or proposition) as true
disbelieve vb vt : to hold not to be true or real : reject or withold belief in vi : to withold or reject belief
Note that again, both strong (1b) and weak (1a) atheism are included in the definition.
http://www.infidels.org/library/modern/mathew/sn-definitions.html