What are we counting as a dictionary? One might go to dictionary.com and find "
the doctrine or belief that there is no God," which is it's first definition, or maybe the second one it has "
disbelief in the existence of a supreme being or beings." I guess one might answer "yes" or "no" based on which one they chose, but which one should be chosen or is correct is another discussion.
One might go to the online Cambridge dictionary if one likes that university prestige on their sources, and find their only definition which says "
the belief that God does not exist."
I personally would count something like the SEP or IEP, although they are explicitly encyclopedias, to be useful for being a dictionary on positions, and they both give a multitude of definitions including but not limited to: "
It has come to be widely accepted that to be an atheist is to affirm the non-existence of God. Anthony Flew (1984) called this positive atheism, whereas to lack a belief that God or gods exist is to be a negative atheist. Parallels for this use of the term would be terms such as “amoral,” “atypical,” or “asymmetrical.” So negative atheism would includes someone who has never reflected on the question of whether or not God exists and has no opinion about the matter and someone who had thought about the matter a great deal and has concluded either that she has insufficient evidence to decide the question, or that the question cannot be resolved in principle." There is also "
If, however, “atheism” is defined in terms of theism and theism is the proposition that God exists and not the psychological condition of believing that there is a God, then it follows that atheism is not the absence of the psychological condition of believing that God exists (more on this below). The “a-” in “atheism” must be understood as negation instead of absence, as “not” instead of “without”. Therefore, in philosophy at least, atheism should be construed as the proposition that God does not exist (or, more broadly, the proposition that there are no gods)."
Take your pick man. With the IEP/SEP quotes I bolded the parts I consider definitional there.
And if you prefer physical book dictionaries I don't own any, I'm totally digital in this, sorry about that.
Edit: you asked for links too, so here are links, hope they work...
Definition of atheism | Dictionary.com
atheism
Atheism | Internet Encyclopedia of Philosophy
Atheism and Agnosticism (Stanford Encyclopedia of Philosophy)
Second edit, adding even more dictionaries which say it is a "belief" or include as one definition that it is a "belief" (although this does not answer the question of how we should define belief):
Chambers – Search Chambers
atheism | meaning of atheism in Longman Dictionary of Contemporary English | LDOCE
atheism | Free On-Line English Dictionary | Thesaurus | Children's, Intermediate Dictionary | Wordsmyth
atheism - Wiktionary