A musical instrument that is struck (or sometimes shaken or scraped) to produce sound. This category includes instruments whose own hard substance is made to vibrate (idiophones) and instruments that include a tight membrane that vibrates (membranophones). Percussion instruments may produce tones of definite or indefinite pitch. Their primary function is often rhythmic, but many are used as melody instruments. They include the bell, carillon, cymbal, drum, dulcimer, gamelan, glockenspiel, marimba, piano, steel drum, tabla, tambourine, timpani, vibraphone, and xylophone.

Some percussion instruments can play melodies. These are called tuned percussion. The xylophone, glockenspiel, marimba, vibraphone,and timpani are all tuned percussion instruments.

Other percussion instruments that do not produce a definite pitch are generally termed unpitched percussion. These include most drums (snare drum, bass drum, drum set, etc.) and also other instruments used mostly for special effects. Some of these instruments which add colour to an ensemble are the triangle, gong, castanets, rattle, cowbell, woodblock, tambourine, maracas, claves, and whistles.

The word, "percussion", has evolved from Latin terms: "percussio" (which translates as "to beat, strike" in the musical sense, rather than the violent action), and "percussus" (which is a noun meaning "a beating"). As a noun in contemporary English it is described at Wiktionary as "the collision of two bodies to produce a sound". The usage of the term is not unique to music but has application in medicine and weaponry, as in percussion cap, but all known and common uses of the word, "percussion", appear to share a similar lineage beginning with the original Latin: "percussus". In a musical context then, the term "percussion instruments" may have been coined originally to describe a family of instruments including drums, rattles, metal plates, or wooden blocks which musicians would beat or strike (as in a collision) to produce sound.