Who gave the word adultery its definition?

1) "adultery
"voluntary violation of the marriage bed," c.1300, avoutrie, from O.Fr. avoutrie, aoulterie, noun of condition from avoutre/aoutre, from L. adulterare "to corrupt" (see adulteration). Modern spelling, with the re-inserted -d-, is from c.1415 (see ad-). Classified as single adultery (with an unmarried person) and double adultery (with a married person). O.E. word was æwbryce "breach of law(ful marriage).""
Source and further information:
http://www.etymonline.com/index.php?search=adultery&searchmode=none


2) "The word adultery originates not from "adult," as is commonly thought in English-speaking countries, but from the Late Latin word for "to alter, corrupt": adulterare.

Adulterare in turn is formed by the combination of ad ("towards"), and alter ("other"), together with the infinitive form are (making it a verb). Thus the meaning is literally "to make other." In contrast, the word "adult" (meaning a person of mature years) comes from another Latin root, adolescere, meaning to grow up or mature: a combination of ad ("towards"), alere ("to nourish", "to grow"), and the inchoative infix sc (meaning "to enter into a state of").

The application of the term to the act appears to arise from the idea that "criminal intercourse with a married woman . . . tended to adulterate the issue [children] of an innocent husband . . . and to expose him to support and provide for another man's [children]".
Thus, the "purity" of the children of a marriage is corrupted, and the inheritance is altered. The law often uses the word "adulterate[d]" to describe contamination of food and the like."
Source and further information:
http://en.wikipedia.org/wiki/Adultery


3) The word existed in Old French before (11th century):
"XIe s.
Citation : Si le pere truvet sa file en avulterie en sa maisoun.... , , L. de Guill. 37"
Source and further information:
http://www.mediadico.com/dictionnaire/definition/adultere/1

It's a Latin word, coming from the root of adulteration, "adulterare" to alter, corrupt or mixing of something. The ancient Greek word was "moicheia" and it meant illicit sexual conduct of a married person, or with a married person.

Who defined the word; word? Not sure if you are looking for the root or the actual coining of the word. There is a whole field of study that is concernened with just this that may just drive you crazy. Wish I could be more help - wish I could have been any. +5