A countertenor is a male voice with a range above that of a tenor. It was formerly called alto, but that word now appears to apply only to women.
This often requires the use of falsetto voice. One can reach the countertenor range if one has an unusually high voice, but even then will probably need some falsetto. One could also be a castrato, but that's a different story...
(Sometimes hyphenated, 'counter-tenor'.)