A mezzo is the same as a mezzo-soprano. mezzo forte/piano etc not too loud, softly etc - used in instructions for performing music (from medius; MEDIUM)
A mezzo-soprano is a female singer who sings with a higher range than a contralto but a lower range than a soprano. She became a professional mezzo-soprano. her remarkable mezzo soprano voice. a female singing voice that is lower than a soprano but higher than an alto, or a woman with a voice like this