Am just trying to make a neutral point here, rather than make a point about which definition is best.
A definition that appears in a dictionary signifies that it forms part of 'common usage', absolutely nothing else. Common usage does not imply traditional, long term, uncontested etc. usage.
Gods, for example, means more than 1 god, or the gallery in a theatre. Neither can be said to be a more correct definition than the other, although one can certainly be said to be an older usage than the other.
This is usually not a problem when the 2 definitions refer to completely different things, as in this case, however it becomes more problematic when multiple definitions become applied to the same word (fascist, terrorist, atheist, etc).
A word probably does not start with 2 definitions, it is unlikely that in a word with more than 1 definition, both are equally old. Saying a definition is older also doesn't mean it is the most common or most 'correct' usage either, just that it is older.
For people to say atheist is defined in the dictionary as 'a lack of belief in god(s)', says nothing other than this definition is in common usage. To then use this dictionary definition to state 'the dictionary says absence of belief as one definition, so you can't disagree with this definition' is not true. Being in the dictionary does not mean 'uncontested', 'logically correct' or anything else.
The older definition of atheist is 'someone who believes there are no god(s)', there is plenty of evidence to support this view - look at older dictionaries, or dictionaries with detailed etymologies if you doubt this.
The newer definition is 'absence of belief in gods', probably dating from around the time of the enlightenment.
So how did this become part of 'common usage'?
I would guess (reasonably confidently) that this definition was created by atheists/sceptics for philosophical/ideological reasons, and was used by atheists as part of their debates and arguments. Over time, those who were in agreement with this philosophical position adopted this definition and as atheism grew in popularity it was widely enough used to become part of 'common usage'. I would guess that it has only been included in dictionaries relatively recently, almost certainly 20th C and possibly mid/late20th C (if anyone can find an older dictionary definition than this I would be interested to see it, please share).
In effect, this is now a correct definition purely because atheists have used it to support their ideological position. If enough people use freedom to mean slavery it will become part of common usage and will be listed in dictionaries.
"If we use our definition enough, it will be appear in the dictionary" is thus a factually correct statement.
"If we use our definition enough, you must accept it because it appears in the dictionary" is obviously not a valid argument in light of the previous statement.
Again, you can agree with all of the above, but still think that 'absence of belief' is a better definition than 'belief there are no gods' or vice versa.
Does anybody disagree with any of these points?