Not known; unfamiliar: a modern-day problem unknown in earlier times. Not identified or ascertained: received flowers from an unknown admirer. Not established or verified. Not well known or widely known: an unknown artist. A person or thing that is unknown: "the abyss of the unknown\u201d ( Helena Petrovna Blavatsky). A person who is not well known, as to the general public: cast an unknown in the starring role. Mathematics A quantity of unknown numerical value.