According to the way that most people use the word "atheism", it is not a position of mere lack of belief in gods. I would call failure to have a belief in gods "nontheism", not "atheism"....
Citation needed.
That said, you will find a lot of dictionaries that use the "lack of belief" definition
Oh! There it is!
Bunyip, I've only known you a short while and I already love you like a brother-and/or-sister-but-demographically-most-likely-brother, but you've let a bunch of schoolchildren bat you around like a shuttlecock for going on five pages now. You've let them define every term, fix every goalpost, and set every standard. Enough playing defense, ya?
Some people require a
reason to believe something. If a person has no reason to believe in invisible, weightless fairies, he lacks belief in invisible, weightless fairies---he does not have to be
ignorant of the
idea of invisible, weightless fairies, he merely has to lack a reason to believe in them. If a person has no reason to believe in very sneaky unicorns, she lacks belief in very sneaky unicorns---she does not have to be
ignorant of the
idea of very sneaky unicorns to lack belief in them.
Now: If,
by the very same logic, a person has no reason to believe in a very sneaky
god, you've got yourself an atheist.
It really is that simple.
This whole "are babies atheists?" is an asinine red herring: Yes, babies have no reason to believe in god, and are thus
technically atheists. They also have no reason to believe you'll reappear during a game of peek-a-boo, and thus may burst into tears and soil themselves when subjected to your unexpected disappearance. In neither case do babies contribute anything to the discussion, which is why atheists don't invest a great deal of time in belabouring this ridiculous semantic minutia.
Absent a reason---
the default position---a person does not believe. This is true of
literally everything.
Literally. Everything. Including, for example, the belief
this person has that it's absolutely, unequivocally insane that he's currently typing the one-thousand-nine-hundred-and-nineteenth post in this ongoing saga of monumental absurdism.
Atheism is, by relevant authority
(see Dawkins, Harris), by logic, by
literal dictionary definition, the lack of belief in gods.
Period.