A contralto is the female singing voice in the lower ranges possible for women. This classification is used for those playing witches and bitches in opera and other stage productions, distinguishing them from the soprano heroines who get to sleep with the producers.
This normally ranges from F below middle C to D below middle C. There are contraltos that can reach even lower notes, but they may be large frogs in disguise, transvestites or just may have a bad cold.
The majority of female humans start out as squeaky eardrum-shattering sopranos, gradually descending into contralto ranges as time goes on. Thank God, as listening to little girls sing like rusty hinges has driven adults and animals insane. This rate of change can be very different from person to person. It is unknown if this also happens to singing female dogs and parrots but this should be determined after the next US census is completed. And as professional singers quickly find, there are loads of sopranos as competition in their teens to the late twenties, who then become competition as contraltos later on. This means that contraltos also have difficulty getting work except in dubbing vocals for boy bands that can't sing but still have dance moves developed from playing Dance Dance Revolution.
Well-trained older contraltos can accompany themselves with the clattering of their dentures. Holding proper pitch is another matter entirely.