Percussion is commonly referred to as "the backbone" or "the heartbeat" of a musical ensemble, often working in close collaboration with bass instruments, when present. In jazz and other popular music ensembles, the pianist, bassist, drummer and sometimes the guitarist are referred to as the rhythm section. Percussion instruments are classified by various criteria sometimes depending on their construction, ethnic origin, function within musical theory and orchestration, or their relative prevalence in common knowledge. 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 in 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 musical instruments including drums, rattles, metal plates, or blocks which musicians would beat or strike to produce sound.