How does the Buddha define "atheist"?
Atheism falls under ucchedavāda which was vehemently rejected by the Buddha. It is one of two extremes that Buddha avoided by revealing the middle way; Pratītyasamutpāda.


  "'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:.."
  -Kaccayanagotta Sutta