What's a definition of "simulation"?
Bostrom's original paper (which proposed the "simulation hypothesis") doesn't define it properly that I can see.
It's a very late C20 / early C21 computer technology phrase - because we make models and simulations, and nest things in our computers, we think it is a meaningful concept about our universe.
How would we know our reality is a simulation, vs. e.g. an "alternate dimension"? Or something much subtler that we have no understanding of?
It won't be a "computer" simulation, and I doubt it'll be running on top of a turing machine. There'll be something more interesting and complex going on - perhaps metaphorically analogous to a simulation, but a richer, more alien thing than that.