[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).

Updated this today. For current runners, the biggest thing to take away from it is:

Hessonite split in Any% and No Major Glitches has changed. Now it's when the screen flashes white after the final hit.

Previously, the rule was to split when you see her fall to her knees, but this is unnecessary and a stupid idea on my part... if you kill her using Garnet+Connie's combo, you have to wait 4-6 seconds for the white screen to go away before you see her on her knees. I've edited all the runs' official times to reflect this new rule--everyone shaved off a couple seconds :)