A common theme throughout the definitions on this page is the primary use of "disbelieve" when defining atheism. Some modern dictionaries drop this, but most comprehensive dictionaries do not. For some reason, however, people simply ignore this word and move right along to the secondary sense of "denial." When we take a closer look at the word "disbelieve," however, we find two senses: an active and a passive.
In the passive sense, "disbelieve" simply means "not believe" — thus a person who disbelieves a claim may simply not accept the truth of the claim without going any further, like asserting the opposite. This is the broadest sense of atheism, lacking belief in any gods. In the active sense, "disbelieve" involves deliberately refusing to believe something (possible reasons might include a lack of evidence or an incoherent claim). Once again, however, this is not the same as asserting that the claim is false and represents a slightly narrower version of weak atheism.
In the active sense, "disbelieve" involves deliberately refusing to believe something (possible reasons might include a lack of evidence or an incoherent claim). Thus, the definition of atheism, dating back at least to 1903 and probably much earlier, encompasses both the "weak" and the "strong" senses of atheism used by atheists today. The same will be true, with minor changes in wording, through nearly all of the dictionary definitions quoted here.
Standard Dictionaries on the Definition of Atheism: