An Etymological Dictionary of Astronomy and Astrophysics

1. A portion or division of a whole that is separate or distinct; piece, fragment, fraction, or section; constituent.
   

2. A section or division of a literary work (Dictionary.com).

Etymology (EN): M.E., from O.Fr. part “share, portion; character; dominion; side, path,” from L. partem (nominative pars) “a part, piece, a share, a division; a party or faction,” related to portio “share, portion,” from PIE root *per- “to assign, allot;” cf. Pers. pâr, pâré “piece, part, portion, fragment;” as below.

Etymology (PE): Pâr, variant pâré “piece, part, portion,” parré “portion, segment (of an orange),” pargâlé, “piece, portion; patch;” (dialects Kermâni pariké “portion, half;” Tabari perik “minute quantity, particle;” Lârestâni pakva “patch;” Borujerdi parru “patch”);
Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;”
Gk. peprotai “it has been granted;” L. pars, as above; Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”