Wiktionary
n. A person or thing whose nature or value is a mystery.
WordNet
n. a factor in a given situation whose bearing and importance is not apparent; "I don't know what the new man will do; he's still an unknown quantity"
a variable whose values are solutions of an equation [syn: unknown]
Usage examples of "unknown quantity".
An unknown quantity as a political leader, the gist of the briefing had been, but a bright and capable analyst.
It is an algebraic axiom, which makes us proceed from a known to an unknown quantity, and not from an unknown to a known.
Chinaman was also 'an unknown quantity,' but the chief trouble on the opening march was caused by the persistently active Christopher, who kicked and bucked the whole way.
If men descended from the apes at an unknown period of time, that is as comprehensible as that they were made from a handful of earth at a certain period of time (in the first case the unknown quantity is the time, in the second case it is the origin).
Youth is the smile of the future in the presence of an unknown quantity, which is itself.
And we just had to cure this patient, because its friends at that time were rather an unknown quantity.