Exodus 20:14, the 7th 'commandment' of the Decalogue.
Does it mean only having sex with someone else' wife or having sex outside marriage?
What is the actual Hebrew word and what possible interpretations could there be?
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The word is sin'af, תִנְאָף. Rashi and Rosh say it refers to sexual relations with a married woman not one's wife. Chizkuni says it refers to any prohibited sexual relations, and, if I understand him correctly, ibn Ezra says the same. S'forno says it refers primarily to the former but also to the latter. (All these sources are in their commentaries on this verse.)
The Hebrew word here is ne'ifa, and it doesn't occur all that often. The standard interpretation means "intercourse between a married woman and a man not her husband." The Talmud observes that just like commandment #6, #7 can warrant the death penalty in theory.
The word is not zonah, which means "stray." That word can either mean prostitution, or a married woman straying from her husband. (When Leviticus prohibits a Kohen from marrying a Zonah, the Talmud says that means a married woman has who cheated with anyone -- even though she's now widowed.) "Don't do zanah" would be directed at a woman only. "Don't do ne'ifa" applies to men and women.
Commentaries such as Sforno (cited above) suggest that the Ten Commandments also signify categories that encompass the entirety of the commandments; thus, the specific commandment was "don't do ne'ifa", but the philosophical category from that illustration is intended to cover all prohibited sexual relations.
While there's this illustrative-category thing, the direct meaning of the seventh commandment is most definitely not prohibiting sex out of marriage. That prohibition would be either Deuteronomy 23:18, according to some; or of post-Biblical nature according to others. (See link for more.)