From the italian 'one string'.
The left pedal on most grand pianos. It moves the hammers a bit to the side, so the hammer only strikes two (one in older pianos, hence the name). strings in the trichord, the set of three strings tuned to the same note. The result is a softer and quieter sound. Regular, upright pianos has a soft pedal with which a similar effect is achieved, but in a different way.

Sources: The Piano Education Page - Kid Q&A,
Music Interpretation and Performance Practice QA,

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