An uncertainty of unknown magnitude, consequence, structure, and probability characteristics, possibly with completely unsuspected existence; something that we don't know that we don't know.

plural of unknown unknown

(sometimes postpositive) Not known; unidentified; not well known.

(algebra) A variable (usually x, y or z) whose value is to be found. / Any thing, place, or situation about which nothing is known; an unknown fact or piece of information.

past participle of unknow

plural of unknown

The condition of being unknown

plural of unknownness

(US) Those US military personnel who have died in service, usually in war, without proper burial.

plural of unknown quantity