Adultery
Infidelity
noun, verbal noun, な adjective, の adjective
If you do not have any ethics you'll probably do something crappy, like commit adultery or infidelity. Usually only unethical people would cheat on their spouse, right?
Unlike 浮気, which can mean cheating in either dating or marriage, 不倫 is strictly adultery in marriage. It sounds way more serious (and has way bigger consequences). Basically, all 不倫 is 浮気, but not all 浮気 is 不倫… Both are bad, but one may come with legal fees.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
どうやら妻が不倫をしているようなんです。
It seems that my wife is committing adultery.