The cello (/ˈtʃɛloʊ/ CHEL-oh; plural celli or cellos) or violoncello (/ˌvaɪələnˈtʃɛloʊ/ VY-ə-lən-CHEL-oh;^[1] Italian pronunciation: [vjolonˈtʃɛllo]) is a bowed (and occasionally plucked) string instrument of the violin family. Its four strings are usually tuned in perfect fifths: from low to high, C₂, G₂, D₃ and A₃. Each string is an octave lower than the viola's four strings. Music for the cello is generally written in the bass clef, with tenor clef and treble clef used for higher-range passages.

Played by a cellist or violoncellist, it enjoys a large solo repertoire with and without accompaniment, as well as numerous concerti. As a solo instrument, the cello uses its whole range, from bass to soprano, and in chamber music such as string quartets and the orchestra's string section, it often plays the bass part, where it may be reinforced an octave lower by the double basses. Figured bass music of the Baroque-era typically assumes a cello, viola da gamba or bassoon as part of the basso continuo group alongside chordal instruments such as organ, harpsichord, lute or theorbo. Cellos are found in many other ensembles, from modern Chinese orchestras to cello rock bands.