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").
Unsung
Part of speech: adjective
Definition: Which has not been lauded or appreciated.Not sung.
Example sentence: One of the unsung freedoms that go with a free press is the freedom not to read it.