Defn: A square cap worn by ecclesiastics of the Roman Catholic Church. A cardinal's berretta is scarlet; that worn by other clerics is black, except that a bishop's is lined with green.
[...] all I could see on the pavement opposite us were a pair of pale-faced priests wearing birettas and a young washerwoman carrying a basket of laundry on her head.
Gyles Brandreth. Oscar Wilde and the Vatican Murders, p.61 (2012)