There doesn't seem to be much consensus here.

Wikipedia uses "worldbuilding": https://en.wikipedia.org/wiki/Worldbuilding, as does Reedsy.

Merriam-Webster hyphenates: https://www.merriam-webster.com/word...world-building

And lots of other random sites use two words.

Is there a definitive form? If not, what form do you prefer? I like hyphenating, personally.