I could make this a discussion about semantics or I could refer to a dictionary, but I think none of that would be very helpful.
A wide array of people
The group of people that identify themselves as atheists is not homogeneous at all, just like the group of theists (there are non-theistic religions, so I did not say 'religious people' here) is not homogeneous, and if you ask a number of them about why they identify themselves as atheists, you find that they might all have very different answers.
Some people just don't care. They haven't really thought about it, but they just don't see much reason to believe in one or multiple gods. These people would be called apatheists (etymology: apathy and theism). In this case, it could probably be defined as lack of belief in one or multiple gods (this is a passive position). This view is often equated to pragmatic atheism, although I have found that pragmatic atheists often care about the question, but for pragmatic reasons (rather than epistemological ones); they might simply think that they're better off without religion (for whatever reason). What unites these two kinds of atheists, is that they are not overly concerned with truth claims. They may and probably will think that it is unlikely that there is a god, but it's not their main concern. These people are often called weak atheists.
Positive or strong atheism is the explicit view that no deities exist. It is the rejection of belief in one or multiple gods and thus an active position. This does not mean that the positive atheist is 100% certain that no God exist (see Bertrand Russell's quote below), but that he thinks it's at least highly unlikely, sees no evidence to the contrary and therefore explicitly thinks that there is no god. These people are, in contrast to weak atheist, very concerned with the question: "does God exist?". They may have other reasons for their explicit non-belief, but the truth about the existence of god is definitely of primary importance for them. So not every non-theist (=someone who is not a theist; i.e. someone who does not believe in at least one god) is a positive atheist. What some atheists find problematic about positive atheism, is that it grants the special consideration to one claim (God exists) over a whole range of other, according to them, nonsensical claims (see Sam Harris' quote).
Even stronger is the view of the New Atheists. They not only reject a belief in God, they reject religion in society and think it should be actively countered. They are antitheist: they think theism is harmful to society. Note that atheism is not a necessary condition for antitheism: you might believe in God, but still think that this belief or at least organised religion is detrimental to society.
There are very few atheists who are absolutely certain that God (or multiple gods) does not exist, but many of them think it's is highly unlikely. As Bertrand Russell said:
“As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an Agnostic, because I do not think that there is a conclusive argument by which one can prove that there is not a God. On the other hand, if I am to convey the right impression to the ordinary man in the street I think that I ought to say that I am an Atheist, because, when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods.” (Bertrand Russell, Am I an Atheist or an Agnostic, 1947)
I think many atheists, ranging from the New Atheists to weak or non-explicit atheism and apatheism, would agree with Bertrand Russell's quote to a large extent. However, I think it would be too narrow and many people who identify as atheists would not fit this definition. What I think is the least we can say, is that everyone who identifies as an atheists must lack a belief in one or more gods; it is thus a necessary condition. Whether it is also a sufficient condition, is a topic for debate, as we've seen.
That does not strike me as a very good definition though as every inanimate object in the universe has a[n] "absence of faith"
This is actually something many atheists would acknowledge; they see no reason to call have a specific word for their religious position, because to them, it is the "natural position". Baron d'Holbach said: " "All children are born Atheists; they have no idea of God."
Sam Harris wrote:
"In fact, "atheism" is a term that should not even exist. No one ever
needs to identify himself as a "non-astrologer" or a "non-alchemist."
We do not have words for people who doubt that Elvis is still alive or
that aliens have traversed the galaxy only to molest ranchers and
their cattle. Atheism is nothing more than the noises reasonable
people make in the presence of unjustified religious beliefs." (Sam
Harris, Letter to a Christian Nation, 2006, p.56)