Accordions (from 19th-century German Akkordeoncode: deu promoted to code: de , from Akkordcode: deu promoted to code: de —"musical chord, concord of sounds") are a family of box-shaped musical instruments of the bellows-driven free reed aerophone type (producing sound as air flows past a reed in a frame), colloquially referred to as a squeezebox. The essential characteristic of the accordion is to combine in one instrument a melody section, also called the diskant, usually on the right-hand manual, with an accompaniment or Basso continuo functionality on the left-hand. The musician normally plays the melody on buttons or keys on the right-hand side (referred to as the manual), and the accompaniment on bass or pre-set chord buttons on the left-hand side. A person who plays the accordion is called an accordionist. The concertina and bandoneon are related, but do not have the diskant-accompaniment duality. The harmoneon is also related and, while having the descant vs. melody dualism, tries to make it less pronounced. The harmonium and American reed organ are in the same family, but are typically larger than an accordion and sit on a surface or the floor.

The accordion is played by compressing or expanding the bellows while pressing buttons or keys, causing pallets to open, which allow air to flow across strips of brass or steel, called reeds. These vibrate to produce sound inside the body. Valves on opposing reeds of each note are used to make the instrument's reeds sound louder without air leaking from each reed block.

The accordion is widely spread across the world because of the waves of migration from Europe to the Americas and other regions. In some countries (for example: Argentina, Brazil, Colombia, the Dominican Republic, Mexico, and Panama) it is used in popular music (for example: chamamé in Argentina; gaucho, forró, and sertanejo in Brazil; vallenato in Colombia; merengue in the Dominican Republic; and norteño in Mexico), whereas in other regions (such as Europe, North America, and other countries in South America) it tends to be more used for dance-pop and folk music.

In Europe and North America, some popular music acts also make use of the instrument. Additionally, the accordion is used in cajun, zydeco, jazz, and klezmer music, and in both solo and orchestral performances of classical music. The piano accordion is the official city instrument of San Francisco, California, United States. Many conservatories in Europe have classical accordion departments. The oldest name for this group of instruments is harmonika, from the Greek harmonikoscode: ell promoted to code: el , meaning "harmonic, musical". Today, native versions of the name accordion are more common. These names refer to the type of accordion patented by Cyrill Demian, which concerned "automatically coupled chords on the bass side".