Piano is defined as the sound level when music is played softly.
An example of a piece that would be described as being played "piano" is a musical piece that is played very quietly.
The definition of a piano is a large keyboard instrument with keys that strike wires which are stretched a metal frame inside a wooden case, or a piece of music that is written to be performed with a soft volume.
An example of a piano is a large instrument that you play by pressing black and white keys that cause hammers to hit wires to produce sound.
A black grand piano.
piano definition by Webster's New World
soft(ly): often used as a musical direction
Origin: Italian soft, smooth ; from Classical Latin planus, smooth, plane
a large keyboard instrument, each key of which operates a small, felt-covered hammer that strikes and vibrates a corresponding wire or set of wires stretched on a metal frame that is enclosed in a wooden case of various forms