Unknown
Part of speech: adjective
Definition: Not known; unidentified; not well known.
Part of speech: noun
Definition: A variable (usually x, y or z) whose value is to be found. Any fact or place about which nothing is known (as in the phrase "into the unknown").
Unfamiliar
Part of speech: noun
Definition: An unfamiliar person; a stranger.
Part of speech: adjective
Definition: Strange, not familiar.
Example sentence: The chief virtue that language can have is clearness, and nothing detracts from it so much as the use of unfamiliar words.