Atheism do not require absolute certainty about the non-existence of God, it can be (and more often is) lack of belief in God. These are sometimes called strong and weak atheism.
I am not 100% certain that ghosts, leprechauns, vampires or invisible pink unicorns do not exist. I merely have no reason to assume that they do exist so I do not believe in them.
Dictionary.com disagrees with you.
the doctrine or belief that there is no God.
2. disbelief in the existence of a supreme being or beings.
a person who holds that the existence of the ultimate cause, as God, and the essential nature
of things are unknown and unknowable, or that human knowledge is limited to experience. Synonyms: disbeliever, nonbeliever, unbeliever; doubter, skeptic, secularist, empiricist; heathen, heretic, infidel, pagan
2. a person who denies or doubts the possibility of ultimate knowledge in some area of study.
3. a person who holds neither of two opposing positions on a topic: Socrates was an agnostic on the subject of immortality.
So, Atheism is a certainty of no God or disbelief of God, (IE: Anti-Theist). Agnosticism is a lack of belief
Dictionary is correct, you interpretation of it is not.
Anti-theism is not same as atheism. It is active opposition of religion, belief that religion is harmful.
The dictionary definition 1 of atheism is sometimes called strong atheism while 2 is weak atheism.
Agnostism means that you admit that you don't know, even that it is impossible to know, that things could be either way.
Now, back to leprechauns.
Are you a leprechaunist, aleprechaunist or agnostic towards leprechauns?