plural of as-yet-unknown

A certainty of unknown magnitude, consequence, structure, and probability characteristics; something that we don't know that we know.

plural of unknown known

An uncertainty of known magnitude, consequence, structure, and probability characteristics; something that we know that we don't know.

(law) An individual yet to identified.

plural of person unknown

The representative of military personnel who have died unidentified without proper burial, usually in war.

(idiomatic) A person or thing whose nature or value is a mystery. / Used other than figuratively or idiomatically: see unknown, quantity.

plural of unknown quantity

a phone number (on a phone call) that is unfamiliar to the recipient / a hidden number; a phone number that is hidden from the recipient