Has anyone taken a stab yet at defining "Core Rulebook"?
tl;dr, I propose one possible definition of "Core Rules" - It's the core conflict resolution mechanism, everything else is GM prep (even when that prep is provided by a publisher)
One way to see this is "Core" means just the rules, whether explicit (how to resolve a skill check); or implicit (how to create/prep a class; followed by how to create a character with the class the GM or a publisher (including WotC) prepped).
Or, we could define this as "all the stuff currently in the 5e MM, PHB, and DMG" - that's core books because that's what WotC calls them.
I mean, is the MM considered core? Or is it just prep created by a publisher? A good chunk of the DMG is optional rules and magic item tables - are they really core? Or just prepped content?
Truly, if we boiled D&D 5e down to just the "rules", I'm pretty sure I could fit that into a single 64-page zine. Actually, maybe even a 1 page pamphlet.
Actually here's an attempt right here
When there's uncertainty in the outcome of any situation in the game, roll a d20. If the roll plus modifiers is higher than the Difficulty Class the GM sets, the outcome goes as the player wanted. If not, the GM gets to apply some sort of penalty (damage, condition, or something else) based on the fiction of the moment or their prep (monster effects, magic effects, et al are part of GM prep). Difficulty Class may also be called Armor Class; or even something else.
Exception: In some cases, based on GM prep, an effect may be something a character (either PC or NPC) wants to resist. In this case, a "Saving Throw" is made, wherein the character rolls a d20, applies modifiers, and if the roll is equal to or higher than the Difficulty Class of the effect (based on GM prep), then the effect is avoided. If it is lower than the Save DC, then the effect is applied
And of course maybe another 10-15 pages devoted to "How to Prep" - like how to create character classes, heritages/races, feats, monsters, backgrounds, magic items, equipment, spells.
Sections on "how to balance"; "how to create encounters"; "how to handle non-combat 'stuff'"; "how to make the game fun"; "how to adjudicate" etc doesn't seem
core to me.
Even some of the stuff in the "Combat" section doesn't seem core - it seems like WotC prepped content based on default setting assumptions about the pseudo-European medieval fantasy setting of Faerun. Take the Jump rules for example. They apply well in the default D&D setting. BUT if you were to prep/create a setting on a low-gravity planet, then the Jump rules would be different.