adultery, criminal conversation, fornication (noun)
extramarital sex that willfully and maliciously interferes with marriage relations
"adultery is often cited as grounds for divorce"
familiarity, intimacy, closeness (noun)
close or warm friendship
"the absence of fences created a mysterious intimacy in which no one knew privacy"
affair, affaire, intimacy, liaison, involvement, amour (noun)
a usually secretive or illicit sexual relationship
closeness, intimacy (noun)
a feeling of being intimate and belonging together
"their closeness grew as the night wore on"