Falsetto, otherwise known as "false voice" is a singing technique that allows men to reach notes and ranges that are commonly sung by women. The name Falsetto, derives from when choirs in the middle ages featured castrated males singing the high parts. The falsetto singing technique was commonly used by European choirs from the 16th to the 19th century. Frankie Valli was singing in Falsetto in the Four Seasons.
Falsetto is unnaturally high-pitched voice especially from a man. It is a characterization of phonation that is produced by the vibration of the ligament filled edges of the vocal cords. Visit en.wikipedia.org/wiki/Falsetto for more information.