Not yet known — nobody has worked out who this person is or what this thing is. It is the word you use when the identity is genuinely still a blank: an unidentified body, an unidentified illness, an unidentified object in the sky (that is where UFO comes from — Unidentified Flying Object). It has a careful, official sound, and it lives mainly in news reports, police statements, and scientific writing. Notice the useful difference from anonymous and unnamed. If a newspaper says an unnamed official, someone knows perfectly well who it is and is choosing not to say. Unidentified means the knowledge itself is missing. So 'an unidentified man' is not being protected — he genuinely has not been identified yet. There is also the softer everyday use, half-joking, for anything you cannot name: an unidentified vegetable in the back of the fridge, an unidentified smell in the corridor. And note the built-in 'not yet': it suggests the work of identifying is unfinished, which is why police say a body 'remains unidentified' — the search is still on.