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.

Is that a female alto singing...? It sounds different... it's probably a countertenor.

Countertenor by Sam October 29, 2004

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.)

Countertenor by Joe_Schmuck February 1, 2017