Most people consider only having sex with a person other than their partner as cheating.
However, cheating is actually different for every relationship. To put it simple, cheating in YOUR relationship is you or your partner doing something with someone else that the other partner would not be OK with.
This is because you or your partner (whoever committed the act) would be breaking the pact (agreement) you and your partner have formed between the two of you, whether the agreement was explicit or implicit.
As I said, cheating is different for every relationship. That is because every partner has their own expectations, so there is no universal list of acts that qualify as cheating.
For example, some people would not be OK with their partner flirting; others won’t have a problem with it. And in some relationships, one can even have sex with several different people and the other partner would be OK with that (as is usually the case with open relationships or those couples who label themselves as “swingers”).
In simple terms: Cheating is breaking the rules of the relationship. Whether it’s polyamory, polygamy, monogamy, a card game, or pin the tail on the donkey, the definition is still the same; cheating means breaking the rules.
Oh yeah, if you need to ask people whether something you did is considered cheating, then likely it is. If you want to know for sure, ask your partner. If you’re not willing to ask your partner, then you pretty much know the answer.