Traditionally a term describing a person who doubts or questions all accepted opinions (in its extreme philosophical form, a person who denies the possibility of knowledge or even rational belief in any specific area). It might best be equated with the simple expression in regard to any concept or datum, “prove it.” The term today is used by some people as a euphemism for a type of “debunker,” specifically a person who attempts to disprove any beliefs, facts, or information that refutes their own belief system.