The number of octaves over which a person can sing; the notes, from lowest to highest that a person is able to produce.
The average vocal range for the four basic voice parts are as follows:
Soprano-from middle C to the E two octaves above; however most sopranos are only capable of reaching the G or A one octave above middle C, and can also reach the G or A below middle C.
Alto-from G below middle C to the E flat one octave above middle C, yet some altos can sing as low as one octave below middle C.
note: while a predominantly female part, there are some men who sing alto, as well as some females who sing tenor
Tenor-technically, from middle C to the C two octaves above, but only because tenor parts are written in the treble clef. In reality, the middle C sang by a tenor is one octave below that of the middle C sang by a soprano or alto. Few tenors, like the sopranos, can actually hit the very high C at the top of their range, but, once again like the sopranos, many can also sing quite below the middle C at the supposed bottom of their range.
Bass-from the D flat nearly two octaves below middle C to the B a half step down from middle C.