From The Art and Popular Culture Encyclopedia
1) A part is a strand or melody of music played by an individual instrument or voice (or group of identical instruments or voices) within a larger work. Parts may be referred to as an outer part (the top and bottom parts) or an inner part (those in between). Part-writing (or voice leading) is the composition of parts in consideration of harmony and counterpoint. In the context of polyphonic composition the term voice may be used instead of part to denote a single melodic line or textural layer. The term is generic, and is not meant to imply that the line should necessarily be vocal in character, instead referring to instrumentation or simply to register.
2) A part also refers to the separate printed or manuscript copies of the music for each individual instrument in an ensemble or orchestra, as distinct from the score, which holds the music for all the instruments.
3) A part in great Highland Bagpipe music is a musical strain or sentence. Usually each part consists of four phrases, either one or two bars long. Several sentences combine to produce a paragraph or complete work or tune.