Sotto voce means under the voice — an instruction to perform very softly, in a hushed, almost whispered tone. It implies not just low volume but a specific quality of delivery: intimate, private, and confidential, as though sharing a secret.
The marking appears in both vocal and instrumental music. In opera, sotto voce passages often convey conspiracy, fear, or deep emotion. In instrumental music, it calls for a subdued, veiled tone quality beyond what a simple piano marking would indicate. The effect creates a sense of closeness between performer and listener, drawing the audience in.
In Verdi's Otello, the villain Iago sings his most dangerous lines sotto voce — the quiet delivery makes his words more menacing than any fortissimo outburst could.