Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A distinguishing character or symbol written directly beneath or next to and slightly below a letter or number.
 adj. Written beneath.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A type of lettering form written lower than the things around it.
 n. A numerical index into an array.
from the GNU version of the Collaborative International Dictionary of English
 adj. Written below or underneath. (See under iota.) Specifically (Math.), said of marks, figures, or letters (suffixes), written below and usually to the right of other letters to distinguish them. See suffix, n., 2, and subindex.
 n. Anything written below.
from The Century Dictionary and Cyclopedia
 Written beneath: as, the Greek iota (
ι ) subscript, so written since the twelfth century in the improper diphthongsᾳ (α%26ι ),ῃ (ηι ),ῳ (ωι ): opposed to adscript (as inἈι ,Ἠι ,Ὠι ). Thisι had become mute by about 200 b. c., and was sometimes written (adscript), sometimes omitted.  n. Something written beneath.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. a character or symbol set or printed or written beneath or slightly below and to the side of another character
 adj. written or printed below and to one side of another character
Etymologies
Examples

Speaking of Rsi, I was amused to learn in my mechanical engineering lecture the other day that "Rsys" sys is a subscript is the numerical expression of the reliability of a system.
Contingency Plans: Fresh Rolls, Pit Dogs, Wet Feet, and Spare Wheels

I will copy text * within a single document* (e.g., symbols with subscripts) and when I paste it within the same document, the formatting (of the subscript) is lost.

Hansen and Quinn give us eight distinct patterns, such as “alpha followed by an epsilon becomes long alpha, alpha followed by epsilon iota becomes long alpha with an iota subscript,” and so on, for eight impossible to memorize (at least for me) rules.

All that is missing for notation is the subscript S on the integration sign to designate surface integral; and this is not particularly unusual or exceptional.

It's simply that Arthur's notation is standard and identical in every respect to what Wolfram does  excepting only the omission of the subscript S on the integral sign to help underline that it is a surface integral.

There's NO notation difference from the Wolfram reference, apart from using "x" rather than "a" as the integration variable to identify a point on the surface, and omitting the subscript "S" on the integration sign.

The new equation editor, subscript, and superscript tools make it possible for math and science teachers and their students to use Google Docs for more of their document creations.

There is a new subscript and superscript tool that can be used in writing chemical compounds and mathematics equations.

The sign outside said WME2—kind of like a vanity plate, but with a little “M” and a subscript “2.”
At Wilshire and Rodeo, Ari of “Entourage” and Ari of “WME2″  Carpetbagger Blog  NYTimes.com

University of Chicago Press 2008 journal subscript ...
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