Technically cheating is only if married but "cheating" is a term often used with boyfriends and girlfriends.
So...your seeing, dating ... or whatever... a girl and you hookup with a different girl is that cheating?...no
Or...you've had a few dates with a girl, you go for a drink with another girl....or other way around....she goes out with another guy...not cheating.
Many many people think if you've been dating or with someone for years...but not married it's cheating....is it?
Now....you are casually dating someone.....or it's more of a sex thing without dating at all.
Before I was married I dated a lot of women. I had several gals I only had a sexual deal with, some were married.
If your spouse is fucking someone else without you knowing that's cheating.