unknown : Idioms & Phrases
unknown quantity
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noun a factor in a given situation whose bearing and importance is not apparent; he's still an unknown quantity"
- I don't know what the new man will do
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noun a variable whose values are solutions of an equation
unknown.
WordNet
unknown region
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noun an unknown and unexplored region
terra incognita; unknown.
- they came like angels out the unknown
WordNet
unknown soldier
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noun an unidentified soldier whose body is honored as a memorial