A “Mosca Bianca” (literally, white fly) is, according to a common metaphor in the Italian language, a person or object with particular characteristics compared to its peers, which make it an extremely rare case.

The metaphor is used with a specific positive connotation, to indicate that the subject is a rare case for the qualities or merits it possesses, as opposed to the expression “black sheep” which instead has a negative connotation.

The expression refers to the fact that common flies are black or dark in color, and seeing a white one would be an extraordinary event.

Similar expressions with the same meaning are frequently encountered in other languages, for example the Spanish perro verde (“green dog”), the French mouton à cinq pattes (“five-legged sheep”), the German weißer Rabe (“white crow”), the English black swan, the Danish hvid krage (white crow).