Contralto

In classical western music, contralto is the lowest female singing voice, and the rarest of the human voice types.

While in common parlance contralto and alto are often used interchangeably, to purists, the word alto, from the Italian for "high", simply describes a range of notes, and by extension the instruments or voice parts that fit this range, whereas contralto qualifies the type of female voice singing within the alto range. So, mezzo-sopranos, countertenors and, in a pinch, even sopranos who can hit low notes and high tenors can sing an alto part in, for example, four-part (SATB) choruses.