That's the common conception. The real one is that atheism is the absence of belief in one particular deity.
Like I said, if you are theist about a particular deity/set of deity, you are automatically atheist of other deity/set of deity.
After all, there is thousands of god in beliefs all over the world, believing in one means that you don't believe in others.
Atheist is the state of the absence of belief toward a specific deity/set of deity.