Some consider adultery as a potentially dishonest sexual act, and use verbs such as commit. ie She committed adultery. Some consider adultery in the same sense as a greeting or a handshake, and simply behaving like an adult. Hence the word ADULTery.
The ultimate act of betrayal and dishonesty against your spouse by sleeping with someone else or as they say "parking your car in another woman's garage."
As wrong as adultery is, its primary cause is one of the married partners not reciving something they need emoptionally or physically from their spouse.