In Domain-driven design (DDD) it is a logical boundary of a domain.
Core subdomains are more volatile and uncertain than other ones, and because of that it’s harder to make a right decision from the start where to draw a line between bounded contexts. Because of that, it’s more beneficial to start with wider boundaries, and decompose into smaller ones as you gain more domain knowledge.
Always look for opportunities to simplify the models by extracting bounded contexts that are laser focused at solving specific problems. But don’t prematurely decompose the system into the smallest bounded context possible.