adj. 1
Not known; unidentified; not well known.
The concept has existed since times unknown.
n. 1
A variable (usually x, y or z) whose value is to be found.

Related words

Forms

From the opposite of unknown, on the left, to its strongest synonyms, on the right.

mysterious A mysterious spaceship was flying away from Earth.
unfamiliar How do you handle unfamiliar or spicy local foods?
undiscovered Ziri wanted to explore the undiscovered areas of the Amazon with Rima.
anonymous "John Doe" is a nickname for an anonymous person.
unidentified Have you ever seen an unidentified flying object?

Use in printed books, per decade from 1850 to 2019.

4 letters: dark¹
5 letters: alien³
6 letters: secret², hidden², occult², unsung²
7 letters: strange¹, obscure², unnamed², unnoted
8 letters: stranger², nameless², pathless
9 letters: anonymous⁴, uncharted³, incognito⁴, unbeknown³, unhonored
10 letters: mysterious⁴, unfamiliar⁴, unexplored³, inglorious⁴, unrevealed, unhonoured, renownless, unrenowned
11 letters: unexplained³, unbeknownst³, unsuspected⁴
12 letters: unidentified⁵, undiscovered⁴, unrecognized⁴, unrecognised
13 letters: unascertained, unapprehended
14 letters: uninvestigated
15 letters: undistinguished⁴

The small number after each word is its number of syllables (alien³ = 3 syllables).