Let us all then agree that the definition of atheist is a person who is not a theist and doesn't believe gods exist. It will only take a minute.
Let us agree that the definition of an atheist is someone who believes no god exists. It will take no longer and doesn't involve the kind of ambiguities or problems with your proposal. Of course, I'm not suggesting we actually agree on some "real" definition for the sake of expediency, as it takes no time at all to agree on an even clearer definition of atheism such as "an atheist is a rational number" or "an atheist is a member of any set which is not the empty/null set" or similarly ridiculous definitions.
The idea is not to argue that the word itself (which is, after all, English, and therefore obviously irrelevant to millions of individuals who don't speak English but do have similar terms in their languages) has some set definition, but
1) The paradoxes and problems inherent in defining ANY default epistemic position, as to the extent such a thing is possible the default is complete ignorance.
2) The fundamental disparity between the beliefs of ALL self-identified atheists and infants
3) The worthlessness of a concept that describes those who "lack a belief" (which is why we don't have words to describe such positions: there is no word for "lack a belief in unicorns", "lack a belief in fairies", "lack a belief in string theory", "lack a belief in the Copenhagen interpretation of quantum mechanics", "lack a belief in the axiom of choice", etc.