How about you read the American Heritage definition again:
Disbelief in or denial of the existence of God or gods.
Note the use of the conjunction or, it indicates an alternative; and also note that disbelief is the first part of the definition. There is nothing "special" about the definition i am using.
All things relavent, it is a good question.
Humans tend to define things in a discrete sense: This or that, believer or non-believer... etc.
