Noun
(idiomatic) A person or thing whose nature or value is a mystery.
Antonyms: known quantity, household name
Used other than figuratively or idiomatically: see unknown, quantity.
Source: en.wiktionary.orgIf with open mind one reads and observes industriously and long; if in so doing one covers a wide field and so covering reflects in terms of realism, he is likely, soon or late, to be brought to a sudden consciousness that Man is an unknown quantity and his existence unsuspected. Louis Sullivan
In any given society the Remnant are always so largely an unknown quantity. Albert Jay Nock
Although the visible buildings give an impression of an organic whole, it is certain that an unknown quantity of additional structures had already been lost to sea erosion before the site's rediscovery and subsequent protection by a seawall. Source: Internet
The site remained undisturbed until 1913, when during a single weekend the site was plundered by a party with shovels who took away an unknown quantity of artefacts. Source: Internet
For example, if series of 10 measurements of previously unknown quantity is performed in laboratory, it is possible to calculate resulting sample mean and sample standard deviation, but it is impossible to calculate standard deviation of the mean. Source: Internet
Real number operations can be extended to imaginary and complex numbers by treating i as an unknown quantity while manipulating an expression, and then using the definition to replace any occurrence of i 2 with −1. Source: Internet