A mezzo-soprano is a woman’s singing voice that sits in the middle: lower than a high soprano, higher than an alto. People also call a woman with that voice a mezzo-soprano, or just a mezzo. Many opera parts are written for this voice — it can sound warm and strong, not only high and bright. This is music-school and opera language. Though actually, in a school choir the lower women’s part is often just called alto, and some mezzo-sopranos sing it. In opera, alto (or contralto) is a still lower, rarer voice. Here mezzo means “middle,” not “not too loud.”