a·the·ist ˈāTHēəst/ noun noun: atheist; plural noun: atheists a person who disbelieves or lacks belief in the existence of God or gods. “he is a committed atheist” synonyms:nonbeliever, disbeliever, unbeliever, skeptic, doubter, doubting Thomas, agnostic; nihilist “why is it often assumed that a man of science is probably an atheist?” …