What is exactly meant by this term? Yes, games have system, we know this, but what is system and what isn't system? For most people, when they say "System", they mean resolution mechanic. To them BRP is percentile roll under Target Number equal to skill. Yep! That's system!
So any game that uses percentile roll under is ripping off BRP, but that only measures chance of success. What about damage/quality of success? Is that part of the system? Uh, yeah, that's system too. How damage is measured is also part of it.
But then, what about initiative? Is initiative part of the system? Umm, OK, sure! So system is chance + quality + initiative! That works, right?
Well what about chargen? Random roll, point allocation, random lifepath, determined lifepath, write down key words, draw a picture - these things are part of the system, right? Umm, yeah - I guess so. Traveller wouldn't be the same without random lifepath. OK, so Chargen is in!
How about advancement after character generation? Earn XPs for killing things? For treasure? Every session? Check off a skill you use? Or maybe one you fail at? Advance over time? Advance as the story calls for it? Is this System too? Yeah, this is probably system too. This is a lot more complex than you might think!
OK, how about Situational Modifiers? Personality mechanics? Attacks of Opportunity? Range zones? Sensory/Notice checks? Hit Location? Vehicle rules? Shut up! Shut up! Yeah, these are system too. It all affects the flavor of the game.
So if I change one of these things, like the vehicle rules, I'm changing the system? Yeah - no! You're not! Vehicle rules are peripheral!
OK, so some parts of system are more equal than others? Yeah! Got it in one!
So, what can I not change and still have the same system? Uh, the resolution mechanic? That's core!
OK. If I change the resolution mechanic, but leave everything else the same, it's a different system? Yeah! Right! I think!
Then the resolution mechanic defines the system. Yep! Wait! No! Crap! I'm back at the beginning again!