Hm, I didn’t think of it that way; of course our defninitions may differ somewhat.
This page (http://www.religioustolerance.org/isl_adul1.htm) seems to say that adultery is “non-marital sex.”
Apparently the latter. “Adultery” would necessarily be an approximate translation, which apparently here means just plain “illegal sex”. In the Judeo-Christian tradition it has come to mean sex where one of the parties is already married to someone else; IIRC the earlier meaning was sex between people who could not legitimize the union by marrying (the usual reason being that someone was already married).
I read somewhere that the so-called Islamic custom of stoning adulterers was actually an Islamic concession to Judaic law, some centuries ago.
Don’t recall where I read it but probably on-line. (I tend to remember where I read things if I read them in print rather than pixels.) This was some months ago–when the Miss Universe pageant was pulling out of Nigeria in protest of stoning some woman for adultery.