Part (music)
Sometimes music for the piano or organ may be written in a strict number of parts, e.g. a 5 part fugue for the piano could also be performed on five different instruments because there are always exactly five different lines of music (although sometimes a part will have a rest, but this is always shown by rests in the music).  Although written on two staves for the pianist (or three staves for an organist) it could be written on five staves: one stave for each part.