Cheating is breaking an existing agreement or vow and keeping/intending to keep that breech of the agreement a secret from the person or people with whom it was made rather than informing the other(s) that the agreement is over (which is "leaving").
Ethical nonmonogamy is
not cheating. (ENM may also be known as “consensual nonmonogamy” or “disclosed nonmonogamy.”)
For example, Mark and Jenny are
swingers. Their agreement is that they will only be with others when they are both present. Anything more than a hug or kiss on the cheek with someone else, or revealing genitals to someone else in a social context, when they are not together, is a no. They attend parties together and have sex with others there; that's not cheating. But while Jenny is out of town on business, Mark picks up a woman at a bar and she gives him oral sex. Under the agreement he has with Jenny, this is cheating. Other swingers have different rules.
Some people have open relationships/marriages and a "don't ask, don't tell" policy, meaning the partners can be with others and won't ask or tell each other about who or what or when, often with certain conditions such as using protection and having STI tests. If you're not communicating with a potential partner's other partner(s), you can't be sure that being with you wouldn't involve your potential new partner cheating. Ultimately, though, at some point you just have to trust what someone tells you because establishing an absolute negative is extremely difficult. How can you be sure they don't have an agreement with some other person out there that would be breached with whatever you're doing? You can't. You can only be sure that
you're not cheating on someone.*
Ethical nonmonogamy can take many forms, from lifelong spousal triads and quads (polyamory) to couples swapping to swinging to open marriages to couples engaging in the occasional casual threesome to someone living alone but having casual hookups.
Informed consent is the core of all of this. While ENM is not cheating, people who profess or attempt to maintain ENM might cheat, just like monogamists might cheat. If there is a closed triad and someone in the triad has a secret lover on the side contrary to the agreement with the two others in the triad, that's cheating. "Fluid bonding" is a term used in polyamorous circles. Someone might only have unprotected sex with one person, and protected sex with others. Going without protection would be a form of cheating.
One should never assume that someone who is married or at least coupled isn't available, although one can certainly decide they don't want to be with someone who is married or coupled. That's each person's decision to make. Just because someone is married or has a partner doesn't automatically mean they aren't available for romance or sexual affection or some other social activities (meals together, going to the theatre).
While one may decide that kissing someone else is cheating and a deal breaker for their relationship,
not all cheating has the same implications and risks.