The below is from an article on the meaning of the word "atheism," on a site called "Atheist Revolution." It was written because many atheists don't know how to define themselves:

"Atheism comes from the Greek 'a - theos,' and since the 'a' prefix means 'without' or 'the absence of,' we must first make sure we understand theism. Theism refers to the belief that some sort of god or gods exist. A theist is one who accepts the theistic claim (i.e., some sort of god or gods exist). An atheist is one who does not accept the theistic claim. That is, atheism means 'without theism' and refers to the absence or lack of theistic belief."1

So "atheism" means "without a belief in God," and an "atheist" is someone who is "without God." The word comes from the Greek word "atheos" which means among other things "without God."

So if you're an atheist, you can tell people that you're without God. Now you know how to define yourself.