Actually if we are going to use the dictionary, it is exactly: ''disbelief or lack of belief in the existence of God or gods.''
That definition is fine, but atheists are human beings too. They also have their own ideas about the origins of life, the universe, consciousness etc. These ideas are "beliefs", aren't they?
We all have our own beliefs, what's your point? Religious people are no longer religious because they also use or understand science? The point is that atheism itself is a disbelief or lack of belief, it's simply not accepting unproven facts, religious people themselves are atheists, they reject the other thousands of gods.