- a person who questions the validity or authenticity of something purporting to be factual.
- a person who maintains a doubting attitude, as toward values, plans, statements, or the character of others.
- a person who doubts the truth of a religion, especially Christianity, or of important elements of it.
- (initial capital letter) Philosophy.
- a member of a philosophical school of ancient Greece, the earliest group of which consisted of Pyrrho and his followers, who maintained that real knowledge of things is impossible.
- any later thinker who doubts or questions the possibility of real knowledge of any kind.