As of definition, my favourite one (obviously influenced by my favourite kind of games) is "it's like watching and shooting a film at the same time".
But as of expaining what RPGs are to clueless people, I just... Run them an RPG.
– RPGs? What's that?
– OK, imagine: you are leaving for work in the morning, you unlock your front door and... It doesn't open. You apply more force, but the damn thing just won't budge. What ya gonna do?
– Uhm, I don't know, I'm gonna call my boss to warn them I'll be late and call a locksmith?
– You pull out your phone. No signal. And you start hearing things. Whispers? You can't exactly place the language, but those sound like words. So, what ya gonna do?
and this goes on for several minutes, and then I wrap up: "So this is more or less how RPGs are played, but we'll also have rules to make communication and stuff easier and to provide prompts to jump off from".