What Things Are Considered Cheating?
What Is Cheating?
Cheating in this context is any way you would commonly be considered to have cheated on your spouse. This includes sex but isn't limited to sex. What is considered cheating to most people?
Sex Outside of the Relationship
Sex in any form outside of your relationship is cheating. The standard definition of cheating is full sexual intercourse. Cheating you your partner also includes sexual release - whether oral or another way.
This is why phone sex or sexual discussions with someone online is considered cheating. You were seeking sexual release with someone who wasn't your partner - technology wasn't a layer of protective insulation from the term cheating, it was an enabler to this new way you could cheat on your partner. For example, getting your kicks from a girl on a webcam while your partner is asleep is cheating - it is simply facilitated by the camera.
There are jokes and horror stories of sending dick pictures and sexy selfies to the wrong person. If you are sexually enticing someone who isn't your partner in this way, you are cheating - because it is an invitation to sexual involvement and you're already involved.

Emotional Intimacy
Emotional intimacy with someone other than your partner is cheating. This doesn't make friendships with someone else cheating. It is when you tell the other person things that your partner should know - and you aren't telling your partner these things - that you've undermined the relationship.
There is an old joke that a woman's desire for emotional intimacy often leads to sex while a man's desire for sex sometimes leads to emotional intimacy. Women are hardwired to fall in love with sexual partners and desire sex with the person they are emotionally intimate with. This is why a man having a one night stand is less of a threat to a relationship than a woman becoming emotionally intimate with someone else - she's more likely to leave because of the emotional entanglement with someone else.
Conversely, a man who has repeatedly cheated sexually on a partner has already abandoned the relationship because he isn't keeping his word to be sexually faithful - and a man's emotional intimacy means the official relationship is doomed, because it is hard for men to become emotionally intimate with anyone. Thus the man has not only cheated but endangered his official relationship by becoming emotionally intimate with another woman, and he has almost certainly doomed it if he's both physically and emotionally intimate with someone else.
Wanting to Get Your Ex Back
There are an amazing number of books, articles and videos on how to get your ex back. One of the tips these sources give is to make your ex jealous by being with someone else, then teasing, flirting and reminding them of how much other people want them.
If you are in this relationship to upset, entice and incite your ex-partner, you are cheating on them by betraying the very intention of the relationship, because you are using them to try to get back with someone else.
Betrayals
Many subtle betrayals don't seem like cheating but become so over time. Think of the person who is constantly monitoring what their ex does, tracking their activities and comments online, even while officially with a new person. This undermines the new relationship and is a betrayal of that person's intentions.This is a betrayal even when they aren't actually trying to get the ex back in a relationship but are instead bound in a love-hate relationship. It is hard to be faithful to the new partner when your emotional focus is someone else.
Then there are the actions that someone writes off as not really cheating that are betrayal, like the woman who has sex with her ex to get him to come visit the kids or guarantee child support is paid - betraying her current partner but has a good excuse for doing so.