An Etymological Dictionary of Astronomy and Astrophysics

1. Math.: A number or symbol, often written as a → subscript or → superscript to a mathematical expression, that indicates an operation to be performed, an ordering relation, or a use of the associated expression.
   

2. Physics: A measure of a particular property; e.g. → adiabatic index; → color index; → index of refraction.
   

3. In some → astrolabes, the → rule.
   

4. A sequential arrangement of items, especially in alphabetical or numerical order. → Index Catalogue.

Etymology (EN): Index, from L. index “forefinger, pointer, sign,” literally
“anything which points out,” from indicare “point out, show,” from in- “in” + dicare “to proclaim,” from stem of dicere
“to speak, to say;” PIE base *deik- “to point out” (cf. Av. daēs- " to show;" Skt. dic- “to point out, show;” Gk. deiknynai “to prove;” O.H.G. zeigon; Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).

Etymology (PE): 1) Dišan, from diš-, simple aorist of Av. daēs- “to show,” as above, + suffix -an.
2) Fehrest, from Mid.Pers. payrâstag “arranged,” from payrâstan “to arrange; to adorn” (Mod.Pers. pirâstan); Proto-Ir. *pati-rāsta-, from *pati “to, at, for, in” (cf. Mid.Pers.
pad “to, at, for, in;” Mod.Pers. be, beh;
Old.Pers. paity; Av. paiti “to, toward, in, at;”
Skt. práti; Gk. poti) + *rād- “to direct, arrange”
(cf. Mod.Pers. raj “line, row,” variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst “right, true; just, upright, straight;” Mid.Pers. râst “true, straight, direct;” Sogdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” PIE base *reg- “move in a straight line,” hence, “to direct, rule”).