The English word piano is a shortened form of the Italian pianoforte, derived from clavicembalo col piano e forte ("key cimbalom with soft and loud"). Variations in volume (loudness) are produced in response to the pianist's touch (pressure on the keys): the greater the pressure, the greater the force of the hammer hitting the strings, and the louder the sound produced and the stronger the attack.

The piano was invented in Italy in the early 1700's and has since become one of the most popular instruments in the world.

The standard modern piano has 88 keys, which are arranged in repeating patterns of white and black keys. The white keys represent the notes of the musical scale, while the black keys represent the sharps and flats. When a key is pressed, a hammer inside the piano strikes a string, causing it to vibrate and produce sound. The sound is then amplified by a soundboard, which is a large wooden resonator inside the piano.

A piano has a protective case surrounding the soundboard and metal strings, strung under great tension on a heavy metal frame. Pressing one or more keys causes a hammer made of wood or plastic, padded with firm felt, to strike the strings. The hammer then rebounds from the strings, which vibrate at their resonant frequency. The vibrations are transmitted through a bridge to a soundboard that amplifies the sound by coupling the acoustic energy to the air. When the key is released, a damper stops the string's vibration, ending the sound.

Most notes have three strings, except for the bass, which graduates from one to two. The strings are sounded when keys are pressed or struck, and silenced by dampers when the hands are lifted from the keyboard. Although an acoustic piano has strings, it is usually classified as a percussion instrument rather than as a stringed instrument, because the strings are struck rather than plucked (as with a harpsichord or spinet); in the Hornbostel–Sachs system of instrument classification, pianos are considered chordophones. There are two main types of piano: the grand piano and the upright piano. The grand piano has a better sound and gives the player a more precise control of the keys, and is therefore the preferred choice for every situation in which the available floor-space and the budget will allow, as well as often being considered a requirement in venues where skilled pianists will frequently give public performances. The upright piano, which necessarily involves some compromise in both tone and key action compared to a grand piano of equivalent quality, is nevertheless much more widely used, because it occupies less space (allowing it to fit comfortably in a room where a grand piano would be too large) and is significantly less expensive.