I was reading My Disciple Died Yet Again, and I came across the term "mustard seed dimension". What's that?
A dimension where everything is made of mustard. Mustard people. Mustard animals. Mustard insects. Mustard planets. Mustard stars. Mustard solar systems, etc.
A tiny dimension within the dimension the characters live in. Imagine a small room that can't be seen from outside but can be accesses only when certain requirements are met. Sometimes these dimensions are not inside the main world but are created from "folding the edges" of the main dimension... Or something like that xD Mustard seed must come from the fact that mustard seeds are tiny :/ but that's only my guess. These small/private dimensions are a quite standard element in xianxia stories.
lol, Mustard seed is the Chinese metaphor thingy for something really small. So when they say "mustard seed dimension" it's basically a small spacial pocket with its own world or whatever "dimension" there.
It's a Buddhist reference, something about a Buddha putting an entire mountain into a mustard seed without the mustard seed getting bigger or the mountain getting smaller. There are actually several similar themes (mustard seeds containing large things or entire worlds) in Buddhism, I'm not sure if there's some special significance or if mustard seeds are basically just a stereotypical "tiny thing", like a grain of sand.
This explains it pretty well. It goes on to explain that even a tiny mustard seed can eventually grow into a large plant. In a manner of speaking, the mustard seed had something much larger than itself "hidden within". And then when you think about how that plant will produce seeds, which will grow into more plants, which will produce more seeds... it just goes on and on, exponentially. And all of this originated from a single, tiny seed. There's practically infinite potential "hidden within" it. That's where the idea of "mustard seed dimensions" come from, I'm pretty sure.