First, let’s see check with the Stanford University Encyclopedia of Philosophy.

Excerpt:

‘Atheism’ means the negation of theism, the denial of the existence of God.

Stanford University is one of the top 5 universities in the United States, so that’s a solid definition. To be an atheist is to be a person who makes the claim that, as a matter of FACT, there is no intelligent agent who created the universe. Atheists think that there is no God, and theists think that there is a God. Both claims are objective claims about the way the world is out there, and so both sides must furnish forth arguments and evidence as to how they are able to know what they are each claiming.

Philosopher William Lane Craig has some thoughts on atheism, atheists and lacking belief in God in this reply to a questioner.

Question:

In my discussions with atheists, they  are using the term that they “lack belief in God”. They claim that this is different from not believing in God or from saying that God does not exist. I’m not sure how to respond to this. It seems to me that its a silly word-play and is logically the same as saying that you do not believe in God.
What would be a good response to this?
Thank you for your time,

Steven

And here is Dr. Craig’s full response:

Your atheist friends are right that there is an important logical difference between believing that there is no God and not believing that there is a God.  Compare my saying, “I believe that there is no gold on Mars” with my saying “I do not believe that there is gold on Mars.”   If I have no opinion on the matter, then I do not believe that there is gold on Mars, and I do not believe that there is no gold on Mars.  There’s a difference between saying, “I do not believe (p)” and “I believe (not-p).”   Logically where you place the negation makes a world of difference.

But where your atheist friends err is in claiming that atheism involves only not believing that there is a God rather than believing that there is no God.

There’s a history behind this.  Certain atheists in the mid-twentieth century were promoting the so-called “presumption of atheism.” At face value, this would appear to be the claim that in the absence of evidence for the existence of God, we should presume that God does not exist.  Atheism is a sort of default position, and the theist bears a special burden of proof with regard to his belief that God exists.

So understood, such an alleged presumption is clearly mistaken.  For the assertion that “There is no God” is just as much a claim to knowledge as is the assertion that “There is a God.”  Therefore, the former assertion requires justification just as the latter does.  It is the agnostic who makes no knowledge claim at all with respect to God’s existence.  He confesses that he doesn’t know whether there is a God or whether there is no God.

But when you look more closely at how protagonists of the presumption of atheism used the term “atheist,” you discover that they were defining the word in a non-standard way, synonymous with “non-theist.”  So understood the term would encompass agnostics and traditional atheists, along with those who think the question meaningless (verificationists).  As Antony Flew confesses,

the word ‘atheist’ has in the present context to be construed in an unusual way.  Nowadays it is normally taken to mean someone who explicitly denies the existence . . . of God . . . But here it has to be understood not positively but negatively, with the originally Greek prefix ‘a-’ being read in this same way in ‘atheist’ as it customarily is in . . . words as ‘amoral’ . . . . In this interpretation an atheist becomes not someone who positively asserts the non-existence of God, but someone who is simply not a theist. (A Companion to Philosophy of Religion, ed. Philip Quinn and Charles Taliaferro [Oxford:  Blackwell, 1997], s.v. “The Presumption of Atheism,” by Antony Flew)

Such a re-definition of the word “atheist” trivializes the claim of the presumption of atheism, for on this definition, atheism ceases to be a view.  It is merely a psychological state which is shared by people who hold various views or no view at all.  On this re-definition, even babies, who hold no opinion at all on the matter, count as atheists!  In fact, our cat Muff counts as an atheist on this definition, since she has (to my knowledge) no belief in God.

One would still require justification in order to know either that God exists or that He does not exist, which is the question we’re really interested in.

So why, you might wonder, would atheists be anxious to so trivialize their position?  Here I agree with you that a deceptive game is being played by many atheists.  If atheism is taken to be a view, namely the view that there is no God, then atheists must shoulder their share of the burden of proof to support this view.  But many atheists admit freely that they cannot sustain such a burden of proof.  So they try to shirk their epistemic responsibility by re-defining atheism so that it is no longer a view but just a psychological condition which as such makes no assertions.  They are really closet agnostics who want to claim the mantle of atheism without shouldering its responsibilities.

This is disingenuous and still leaves us asking, “So is there a God or not?”

So there you have it. We are interested in what both sides know and what reasons and evidence they have to justify their claim to know. We are interested in talking to people who make claims about objective reality, not about themselves, and who then go on to give reasons and evidence to support their claims about objective reality. There are atheists out there that do make an objective claim that God does not exist, and then support that claim with arguments and evidence. Those are good atheists, and we should engage in rational conversations with them. But clearly there are some atheists who are not like that. How should we deal with these “subjective atheists”?

Well, my advice is to avoid them. They are approaching religion non-cognitively. When you engage in serious discussions with people about God’s existence, you only care about what people know and what they can show to be true.