Countertenor
The highest male voice; in speaking and singing, countertenors have the normal range of a tenor, but are able, by singing in falsetto, to go higher, into the contralto/alto range, mezzo-soprano range, and, with the great ones, into the soprano range.
They are usually assigned roles written originally for the castratos, yanking those roles away from the contraltos.
They are usually assigned roles written originally for the castratos, yanking those roles away from the contraltos.
Is that a female alto singing...? It sounds different... it's probably a countertenor.
Countertenor
An adult male vocalist who has pretty much an unbroken voice- (whereas, most blokes' voices break at puberty), and who, as a result of this failure to lose his "baby voice" at puberty, remains able to sing in the vocal tessituras- (ranges, to the non-singer), of the female Alto, Mezzo-soprano, and very occasionally, Soprano, without having to resort to the use of Falsetto- (false voice.)
True Countertenors are a very rare thing. Most blokes who think they're Countertenors are actually Falsettists, who sing in the Countertenor registers, using Falsetto- (false voice.)