- 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" (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.