Most Helpful Guy
The intention of being with someone else while currently in a relationship. Usually involves deception and is a intentional act since the person who is cheating isn't mature enough to speak their mind and break up with the person before finding someone else.
Flirting with intent to cheat is cheating
Kissing is cheating
Sex is cheating
The intent is crucial. However, there are some acts that are just cheating wether it was intentional or not.
Sex and kissing (or sending nudes) are the definite forms of cheating.
Texting, talking to other people of the opposite gender isn't cheating but only is when the intern is to cheat, this is very hard to prove. The actions above are what I would look for.
Most Helpful Girl
Two interests at the same time when you are dating or married to one of them.