A countertenor is a type of classical male singing voice whose vocal range is equivalent to that of the female contralto or mezzo-soprano voice types. The countertenor range is generally equivalent to a contralto range, extending from around G3 to D5 or E5, although a sopranist (a specific kind of countertenor) may match the soprano’s range of around C4 to C6. Countertenors often are natural baritones or tenors, but rarely use this vocal range in performance.
A tenor with an unusually high range (as an alto range)
Oxford English Dictionary: A part higher in pitch than the tenor, sung by a high male voice; the alto.
A man who sings regularly in the C3-C6 range. Countertenors are often associated with castrati of the baroque era and many countertenors sing music composed for castrati. Countertenor bodies greatly differ from those of castrati–countertenors do not achieve their vocal ranges through pre-pubescent castration.
Grove Music Online
A male high voice, originally and still most commonly of alto range, though the title is increasingly employed generically to describe any adult male voice higher than tenor. Historically, it derived in England from the contratenor line in late medieval and Renaissance polyphony, via contratenor altus (‘high contratenor’), which – used interchangeably – became ‘countertenor’ and ‘altus’, then alto (as in Italian nomenclature) and, later still, even ‘male’ alto.