A castrato is a male soprano, mezzo-soprano, or alto voice produced either by castration of the singer before puberty or one who, because of an endocrinological condition, never reaches sexual maturity.

Castration before puberty (or in its early stages) prevents a boy's larynx from being transformed by the normal physiological events of puberty. As a result, the vocal range of prepubescence (shared by both sexes) is largely retained, and the voice develops into adulthood in a unique way. As the castrato's body grew, his lack of testosterone meant that his epiphyses (bone-joints) did not harden in the normal manner. Thus the limbs of the castrati often grew unusually long, as did the bones of their ribs. This, combined with intensive training, gave them unrivalled lung-power and breath capacity. Operating through small, child-sized vocal cords, their voices were also extraordinarily flexible, and quite different from the equivalent adult female voice, as well as higher vocal ranges of the uncastrated adult male (see soprano, mezzo-soprano, alto, sopranist, countertenor and contralto). Listening to the only surviving recordings of a castrato (see below), one can hear that the lower part of the voice sounds like a "super-high" tenor, with a more falsetto-like upper register above that.

Castrati were rarely referred to as such: in the eighteenth century, the term musico (pl musici) was much more generally used, though it usually carried derogatory implications ; another synonym was evirato (literally meaning "emasculated").