Tom McGrath defines non-linear structure in this context as intuitively grouped concepts not forming a line or plane in the representation.

“non-linear structure exists and you know we should get into some some nuances on the word linear um before we move off of this topic because there's there's a sort of there's there's a lot of subtlety there um but you know i'm going to say non-linear in the sense of the representations don't form a like the things which are intuitively grouped to us don't form a line or a plane”

“or just a line really um and so this was sort of enough to show that this exists and then the question is like you know i i think whenever you have a supervised method you often want to try and find an unsupervised way of doing the same thing um you know because that lets you answer the question not only does it exist but like what else is there that we might not have expected and how much is there”