Terminology: Why World?

Disclaimer: I am aware that this library is tailored toward games. Moreover, this issue could result in some bikeshedding, so sorry in advance!

Coming from set theory, one thing that feels particularly strange to me is the terminology regarding "world" and "universe". On the one hand, universe is something that is inspired by the universal set, i.e., the set of all sets. On the other hand, world is something very physical (such as a planet), without any correspondence in the set theory. Nevertheless, it is possible to do something very unphysical to worlds, namely merging, i.e., constructing a union of two entity sets. Playing some WoW, it also could be an instance, but then there is no corresponding notion of class (level), which is also not a great choice.

Is there a specific reason why world was chosen over entity set?

Not that there needs to be a specific reason, but programmers already have data structures called 'sets', and there wouldn't be any advantage to conflating an ECS world with a set data structure. Sets do not allow identical elements, while the ECS world does, so it is arguably not a set in the mathematical sense. Universe also has nothing to do with universes from set theory, but more from the cosmological sense of being a shared space in which worlds lie.