Systemic design
Also said as Chemistry engine, Multiplicative gameplay, Immersive sim logic
Building a game from general rules that apply to everything, rather than special cases for each puzzle. If fire burns grass, it burns all grass, whoever lit it, and any solution those rules allow is a real solution.
One new rule multiplies with every rule you already have, so content pays for itself, and players trust the world: if it looks as if it should work, it does. It costs you tight control over what happens.
Say it
“Don't script the ice puzzle. Add one rule: cold freezes water into a walkable surface. Then it also works on rivers, enemy moats and rain, without any more work from us.”
Where you’ve seen it
At GDC 2017, the Breath of the Wild team described a chemistry engine that works out what happens between fire, water, ice, wind and materials. Grass burns, the heat lifts your glider, and metal draws lightning, all from the same rules.
Not to be confused with emergence
Systemic design is how you build. Emergence is what you may get from it. You can also build systems that interact and produce nothing surprising.