Full Definition of FREEMASON. : a member of a major fraternal organization called Free and Accepted Masons or Ancient Free and Accepted Masons that has certain secret rituals.See Freemason defined for English-language learners.ull Definition of FREEMASON. : a member of a major fraternal organization called Free and Accepted Masons or Ancient Free and Accepted Masons that has certain secret rituals. See Freemason defined for English-language learners.