noun.
One who has rejected authority and dogma, especially in religious thinking, in favor of rational inquiry and speculation.
The Century Dictionary and Cyclopedia
noun.
One who is not guided in the formation of his beliefs by obedience to authority, but submits the claims of authority to reason as the ultimate arbiter.
noun.SynonymsUnbeliever, Skeptic, etc. See infidel.
the GNU version of the Collaborative International Dictionary of English
noun.
One who speculates or forms opinions independently of the authority of others; esp., in the sphere or religion, one who forms opinions independently of the authority of revelation or of the church; an unbeliever; -- a term assumed by deists and skeptics in the eighteenth century.