The piano is a musical instrument played by means of a keyboard. It is one of the most popular instruments in the world. Widely used in classical music for solo performances, ensemble use, chamber music and accompaniment, the piano is also very popular as an aid to composing and rehearsal. Although not portable and often expensive, the piano's versatility and ubiquity have made it one of the world's most familiar musical instruments.

Pressing a key on the piano's keyboard causes a felt-covered hammer to strike steel strings. The hammers rebound, allowing the strings to continue vibrating at their resonant frequency. These vibrations are transmitted through a bridge to a sounding board that more efficiently couples the acoustic energy to the air. The sound would otherwise be no louder than that directly produced by the strings. When the key is released, a damper stops the string's vibration. See the article on Piano key frequencies for a picture of the piano keyboard and the location of middle-C. In the Hornbostel-Sachs system of instrument classification, pianos are considered chordophones.

The word piano is a shortened form of pianoforte, the Italian word for the instrument (which in turn derives from the previous terms "gravicembalo col piano e forte" and fortepiano). The musical terms "piano" and "forte" mean "quiet" and "loud," and in this context refers to the variations in volume of sound the instrument produces in response to a pianist's touch on the keys: the greater a key press's velocity, the greater the force of the hammer hitting the string(s), and the louder the note produced.

Much of the most widely admired piano repertoire, for example, that of Haydn, Mozart, and Beethoven, was composed for a type of instrument (the pianoforte) that is rather different from the modern instruments on which this music is normally performed today. Even the music of the Romantics, including Liszt, Chopin, Robert Schumann, Felix Mendelssohn and Johannes Brahms, was written for pianos substantially different from modern pianos.

The piano is a crucial instrument in Western classical music, jazz, film, television, and most other complex western musical genres. A large number of composers are proficient pianistsвАФand because the piano keyboard offers an easy means of complex melodic and harmonic interplayвАФthe piano is often used as a tool for composition. Pianos were, and still are, popular instruments for private household ownership. Hence, pianos have gained a place in the popular consciousness, and are sometimes referred to by nicknames including: "the ivories", "the joanna", "the eighty-eight", "the black(s) and white(s)", and "the little joe(s)". Playing the piano is sometimes referred to as "tickling the ivories".