I've always used something roughly like this...

2 or more people interacting in such a way to cause mutual arousal in your partner, and having the goal for inducing eventual orgasm.

Orgasm is not a requirement in other words, but if it wasn't even a goal, then it's just fooling around. I have debated whether some level of nudity was an element of my definition, or genital contact....but I figured those were needless semantics.