adultery (n.) Look up adultery at Dictionary.com
"voluntary violation of the marriage bed," c.1300, avoutrie, from Old French avouterie (12c.), noun of condition from avoutre, from Latin adulterare "to corrupt" (see adulteration). Modern spelling, with the re-inserted -d-, is from early 15c. (see ad-).

In Middle English, also "sex between husband and wife for recreational purposes; idolatry, perversion, heresy." Classified as single adultery (with an unmarried person) and double adultery (with a married person). Old English word was æwbryce "breach of law(ful marriage)" (compare German Ehebruch). Adultery Dune in Arizona corresponds to Navajo sei adilehe "adultery sand," where illicit lovers met privately.