Nouns

(n)unknown, unknown region, terra incognitaan unknown and unexplored region“they came like angels out of the unknown”
(n)stranger, alien, unknownanyone who does not belong in the environment in which they are found
(n)unknown, unknown quantitya variable whose values are solutions of an equation

Adjectives

(a)unknownnot known“an unknown amount”, “an unknown island”, “an unknown writer”, “an unknown source”
(s)nameless, unidentified, unknown, unnamedbeing or having an unknown or unnamed source“a poem by an unknown author”, “corporations responsible to nameless owners”, “an unnamed donor”
(s)unknownnot known to exist“things obscurely felt surged up from unknown depths”
(s)obscure, unknown, unsungnot famous or acclaimed“an obscure family”, “unsung heroes of the war”
(s)strange, unknownnot known before“used many strange words”, “saw many strange faces in the crowd”, “don't let anyone unknown into the house”