By strict definition of the term, Atheists do not merely have a passive lack of belief in god, but actively deny his existence. The proof for this definition can be found in any reputable defining publication, such as the Merriam-Webster dictionary which simply says an Atheist is one who believes there is no deity.
However, as of late, those who call themselves atheists would have you believe that they simply have no belief in a deity, not that they are making a claim that there is not one. This is obviously a wiser choice, since the existence of a deity cannot be proven or unproven. However, it falls into the definition of agnosticism more than it does atheism. Agnostics simply claim no belief either way...a wise position to take on any fantastical, unprovable claim.
But what is the common ground between agnostics and atheists? Neither believe in a deity. At first, an agnostic might say, "Whoa...I think there may or may not be a god, and am undecided."
Well, if you are undecided, then you do not currently have a belief that there IS a god. So therefore, you don't believe in god.
Atheists don't believe in god. Agnostics don't believe in god. That's the bottom line. We should have a term that encompasses all who do not believe in god. And refer to that collective intelligence as a single group.
Nonbelievers? Unbelievers? Nontheists? Brights? Reasonists?
None of these are very catchy.