[UPDATE - Sep 8, 2018]: I made this post a few months ago outlining split definitions for all categories, but I think it was way too strict. It's better, especially for 101% and CBF% where the gameplay isn't linear, to leave it up to y'all to decide how to divide/organize/label them.

That said, I would like to detail just the specifics for Any%, since that's pretty optimized at this point, and then offer very broad rules for other categories.

(NOTE: the official split rule for Hessonite in Any% and No Major Glitches has been changed!)

Names in parentheses are how I label mine, call them whatever you want obviously:

ANY%

1. (First Fight): "WE DID IT" screen after the first fight
2. (Key Get): "WE DID IT" screen after the fight that gives you Key to the City
3. (Beach City Done): when auto-save icon appears (top right corner) as you enter the woods
4. (Familiar Grounds): when auto-save icon appears as you leave the level
5. (Waterfall Way): when auto-save icon appears as you leave the level
6. (South Trail): when auto-save icon appears as you leave the level
7. (Dave): "WE DID IT" screen after you defeat him (i.e. wait until after the mini-cutscene where Steven says "so cool")
8. (Forge Skip): when auto-save icon appears as you enter Hall to the Heart
9. (Spectral Conclusion): after Light Warrior's final "..." when Steven says "is this what you really want," or i.e. when it falls to its knees
10. (Hessonite): when the screen flashes white after the final hit on her <-- THIS IS NEW

For everything else, basic rule is: if you're leaving a level, split when the auto-save icon appears, and if you're defeating an enemy, split when "WE DID IT" appears. For item-based splits, I think it's best to treat it like you're leaving a level; for example, if, like me, you have a split after getting the Secret Hoozywhatzit for CBF%, I'd split when the auto-save icon appears when you save+quit the game... similarly, if you have a split after gathering a bunch of chroma, I'd split when you leave the zone where the chroma was (i.e. in my 101% run, I end my "yellow+green chroma" split when I enter the Hidden Grove).