adjective
not known
not well-known :not famous
unknown quantity
unknown to
without being known about by (someone)
noun
plural -knowns
the unknown :a place, situation, or thing that you do not know about or understand
[count] :a person who is not famous or well-known
[count] :something that is not known or not yet discovered
[count] mathematics :a quantity that is not known and that is usually shown in equations as a letter (such as x, y, or z)