The accordion is a portable box-shaped musical instrument of the hand-held bellows-driven free-reed aerophone family, sometimes referred to as a squeezebox. A person who plays the accordion is called an accordionist. It is played by compressing or expanding abellows, while pressing buttons or keys, causing valves called pallets to open which allow air to flow across strips of brass or steel called reeds that vibrate to produce sound inside the body.^{[notes 1]}

The instrument is sometimes considered a "one-man-band", as it needs no accompanying instrument; the performer normally plays the melody on buttons or keys on the right-hand manual, and the accompaniment—consisting of bass and pre-set chord buttons—on the left-hand manual.

It is often used in folk music in Europe, North America, Russia, and South America. It is commonly associated with busking. Some popular music acts also make use of the instrument. Additionally, the accordion is sometimes used in both solo and orchestra performances of classical music.