Then I guess it's just a semantic difference in what the word "belief" means, and what you personally mean when you say "I don't believe in leprechauns". I think that if someone has an opinion on the subject, then they believe that opinion to be correct, which naturally implies/requires a belief in the opinion. So when I say "I don't believe in leprechauns", I am also saying "My opinion is that leprechauns don't exist", and since I believe I am correct, I am also saying "I believe that leprechauns don't exist". OTOH, when you say "I don't believe in leprechauns", you mean "there is no evidence to suggest that leprechauns exist, therefore there is no reason to believe that leprechauns exist, therefore I don't believe in leprechauns". I personally think there is no difference, since I don't think many (let alone all, as the definition implies) pure atheists would, if presented with undeniable evidence of God, still believe in no God.