Encounter design
Also said as Combat encounter, Enemy mix, Wave design
Choosing which enemies, how many, where they start and when they arrive, for one fight in one space. The same six enemies can be trivial or terrifying depending on the mix.
Enemy types are words and an encounter is a sentence. Good mixes make one enemy's job the answer to your answer for another, so no single habit works for the whole fight.
Say it
“This fight is six rushers and it is boring: players just back up and shoot. Replace two with long-range shooters on the balcony, so that backing up walks them into fire.”
Where you’ve seen it
Jaime Griesemer called Halo about 30 seconds of fun repeated, and he has said the part often cut off is that those 30 seconds keep changing with new places, weapons and enemies. Doom Eternal's Hugo Martin describes demons as chess pieces.
Not to be confused with difficulty
Difficulty is how hard it is. Encounter design is what kind of problem it is. Doubling everyone's health makes it harder and no more interesting.