Atheist is a non-believer. Prefix “a-“ means absence. Every human is an atheist unless they believe in every god. The word was first used in relation to Christians.

Anti-theist is someone opposed to religion or belief in supernatural. “Anti” means “opposed / opposite to”.

Agnostic is a bullshit cop-out term that at some point in a Christian discourse briefly meant “someone who considers supernatural to not be knowable”, but doesn’t have a proper meaning nowadays. It has a transactional role in conversation - it most often relays unwillingness to continue the conversation on religion.

A “definite belief that there is no god” would be “gnostic atheist” in proper terms. I.e. “god is knowable and he’s absent”. But those proper terms were barely ever alive. Instead, people dance around topic of religion as if it didn’t enjoy enough fucking dances for millennia past.

Atheist really means a definite belief

No.

Interesting what meaning these words have in English.

That's quite specific, tbh. And both are very American in that they are defined around the bed too establish and stand off "sides". As if there is a right or wrong to children believing in a Santa Claus until a certain age, or grown ups believing into an adult variant if that. The morality comes from the actions taken under such a belief.