There are also "sandboxy" games like the open world Zelda, with a chemistry/physics system and multiple-solution puzzles, but wouldn't quite fit the prototypical immersive sim, as there is
still fairly limited NPC interaction. Maybe those count just as systemic games in the wider sense.
I'd hazard that a good summary definition might be "game rules are universally applicable."
In that if there is a rule/mechanism anywhere, it can be used everywhere, with anything responding to it.
Or in Dwarf Fortress + OO/Smalltalk/message terms: every object can and must respond to every message. (to the extent technically possible)
There's a lot of arguing about what constitutes an immersive sim (i.e. NPC interactivity is not one), but the two common concise definitions are "if you can open a door in several inventive ways (including bypassing it) and more generally, if you have so many complex systems in a game that they interact in unexpected ways. Purists also require first-person perspective, but others would also count Hitman or EVE Online.