Adjacent Niche
Also called: niche adjacency, adjacent audience, neighbouring niche
Adjacency is a question about audience overlap rather than subject matter. Two topics are adjacent when the same people already watch both, which is why a woodworking channel can move into workshop tooling and a personal finance channel can move into tax without anything breaking. The Audience tab in YouTube Analytics shows what else your existing viewers watch, and that list is the honest map of where you can go.
The mistake is choosing the adjacent topic by logic instead of evidence. Two subjects that sound related to you can share almost no audience, and a video aimed at one of those lands among strangers while your regulars skip it. Check the overlap before committing a month to it.