As usually atheist is mean a belief that there is no god. Is there any definition by Buddha about atheist ? I read philosophy that inhumanity would called atheist really other than belief in god .
Atheism is a beluef there is no God. Theism is a belief there is God. Buddha was the first inner scientist who asked his disciples not to believe but find out through experience. I think Buddha's position is unique amongst all enlightened beings till his time. His radical departure from belief being the first requirement stands apart. His point was simple - you are miserable and you need to get out.
"'Everything exists': That is one extreme. 'Everything doesn't exist': That is a second extreme. Avoiding these two extremes, the Tathagata teaches the Dhamma via the middle:.."
A language encodes in it an associated culture. Theism/Atheism binary does not apply to the Indic culture. God/no-God was not at all the central question for ancient Indians.
Rather it was who believed in the central philosophy(which may or many not involve God) and who did not. The term used is Āstika & Nāstika.
..the Buddhists themselves have branded only the Cārvākas as nastika. For example, Nagarjuna wrote in his Ratnavali, that nastikya (nihilism) leads to hell while astikya (affirmation) leads to heaven. Further, the Madhyamika philosopher Chandrakirti, who was accused of being a nastik, wrote in his Prasannapada that emptiness is a method of affirming neither being nor non-being and that nihilists are actually naive realists because they assume that things of this world have self-existent natures, whereas Madhyamikas view all things as arising dependently within the context of casual conditions.
There were also Buddhists that were accused of believing in ideas outside of the Buddha's teachings, and they were called nastika in the "Bodhisattvabhumi" (a section of the Yogacarabhumi by Asanga) and the scripture also declared they should be subject to isolation so their views do not infect the rest of the Buddhist community.
Source : Wikipedia - Āstika and nāstika
Buddhism accept there are deities in this sense it is not atheism but hold the position the deities have a finite life like any other being and are not alimity / omnipotence (unlimited power), creator deity, omniscience (infinite knowledge), omnipresence (present everywhere), omnibenevolence (perfect goodness) which slants more towards atheism.