not known; not within the range of one's knowledge, experience, or understanding; strange; unfamiliar.

not discovered, explored, identified, or ascertained

the unknown parts of Antarctica.

not widely known; not famous; obscure

an unknown writer.

a thing, influence, area, factor, or person that is unknown

the many unknowns in modern medicine; The director cast an unknown in the leading role.

Mathematics . a symbol representing an unknown quantity

in algebra, analysis, etc., frequently represented by a letter from the last part of the alphabet, as x, y, or z.

adjective

familiar , identified , known

adjective

alien , anonymous , concealed , dark , desolate , distant , exotic , far , faraway , far-off , foreign , hidden , humble , incog , incognito , little known , nameless , new , remote , secret , so-and-so , strange , such-and-such , unapprehended , unascertained , uncelebrated , uncharted , undiscovered , undistinguished , unexplained , unexplored , unfamiliar , unheard-of , unidentified , unnamed , unnoted , unperceived , unrecognized , unrenowned , unrevealed , unsung , untold , x * , cryptogenic , idiopathic , imponderable , incalculable , incomprehensible , inconspicuous , inglorious , innominate , nobody , noumenal , obscure , unhonored , unknowable