WordNet sense 1 (a stringed instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds):

pianoforte, forte-piano

WordNet sense 2 (chiefly a direction or description in music; very soft):

pianissimo

WordNet sense 3 (used chiefly as a direction or description in music):

soft

WordNet sense 4 (used as a direction in music; to be played relatively softly):

softly