Most of us recognize this type of plot: Bob is married to Alice.One day, Bob sees Carol at a club and is attracted to her.But here's the thing: Bob doesn't tell Alice about it.He doesn't dump her, he doesn't tell her that he thinks the marriage is on the rocks, he doesn't talk to her about Polyamory or swinging, he doesn't even ask for "more space".
There are a lot of names for cheating on your partner, but most of them have the same outcome: a world of hurt.
What cheating usually depends on the context of the story and the characters involved.
Stories aimed at younger audiences, or with a clear emphasis on romantic, monogamous relationships, will probably count kissing and flirting as cheating.
However, it's worth noting that even further back in the day, the gods, goddesses and minor side characters of mythology listed "infidelity" under "Hobbies", didn't particularly care if their new "partner" was willing, and got away with it.
Only their mortal lovers got the nasty side of the wronged wife's/husband's temper when the affair was discovered.
He continues to play the part of her husband, and expects her to continue being his wife, hoping that Alice won't notice when he starts coming in late for dinner, or ask him about the mysterious expenditures on their joint account.