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, http://www.piano.avijon.com/pnokq_a.html
Music Interpretation and Performance Practice QA, http://www.serve.com/marbeth/interpretation_QA.html