Build diversity
Also said as Build variety, Viable builds, Option space
How many different ways of setting up your character or army are actually good enough to use. A game may offer thousands, and only the ones close to the best count.
It is the difference between having choices and appearing to have them. A single overpowered item can quietly wipe out most builds, since everything without it falls below the line, and nobody will mention that in the patch notes.
Say it
“Check build diversity after every patch: count builds within 5% of the top DPS. If that drops below ten, something has become mandatory.”
Where you’ve seen it
Path of Exile's huge passive skill tree is built for this: the same tree has to support very different characters, and its players publish and compare build guides for each new league.
Not to be confused with number of items
Adding items adds possible builds. It does not add viable builds, and it can reduce them if a new item is too strong to leave out.