Atheism is a lack of belief in gods. Atheism is not explicitly present in many games, largely due to the fact that most games do not touch upon religion.
Atheist characters are those who profess a lack of belief in gods. Beyond a simple lack of belief, some characters may also adopt "positive atheism" (the belief that no gods exist). Games such as Neverwinter Nights allow players to create atheistic characters that are immune from various curses but unable to utilize faith-based benefits or select certain character classes such as monks.
Atheism as a contrivance
In addition to atheism in characters, some games adopt an atheistic worldview. In such games, regardless of what characters may believe, gods do not exist.