Literally, it means no mods.

But sometimes performance mods are allowed.

Or sometimes certain launchers are allowed.

Sometimes texture packs aren’t.

Or shaders are.

Or playing with keepInventory on or doFireTick off isn’t.

I have no goddamn clue anymore.

As far as game mechanics, no mods that change gameplay in any meaningful way (including quality-of-life mods) is vanilla.

In terms of tech support, an unmodified base install is vanilla. (This is important because if you are looking for help with a crash for example, knowing you have OptiFine or something is very relevant.)

Define “meaningful way”. What about resource packs? They change how the game looks drastically, but it’s purely visual. What about performance mods? Higher and lower FPS definitely impact one’s movement and accuracy with a mouse.

In my opinion, this is usually how I classify things:

• Vanilla: no mods, resource packs allowed (it just changes visuals anyway, and is included in the base game).
• Vanilla+: may contain mods/datapacks that simply tweak experiences in a way that don’t alter the flow of the game much. Things like Optifine/Iris, Sodium/Rubidium, fast leaf decay, immersive portals, right-click-harvest, mouse/crafting tweaks, and I’d even say player/mob heads. Essentially, doesn’t add blocks, items, etc, but just enhances small parts of the game that you’re already doing in vanilla.
• Modded: adds blocks, items, functionality, or more, beyond the scope of Vanilla+