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An unusual or exceptional person or thing; someone out of the ordinary. It was first used figuratively by the Roman satirist Juvenal.
Rara avis in terris nigroque simillima cycno. "A bird rarely seen on earth, and very like a black swan." Juvenal chose a black swan for his comparison because the Romans had no idea that black swans even existed and so it was considered as impossible as a white crow.
This entry is located in the following unit: Latin Proverbs, Mottoes, Phrases, and Words: Group R (page 1)