A “big ball of mud” is a term used in software development to describe a complex, difficult-to-maintain system that lacks a clear structure or organization. It is often the result of poor design, ad-hoc coding, and lack of foresight. This type of system is hard to change, understand, and test, and can often lead to technical debt and long-term issues for a software project. The term emphasizes the idea that the system has become a “big ball of mud” due to incremental changes and additions, rather than being the result of a well-planned and structured approach.

In contrast, Domain-Driven Design (DDD) advocates for a deliberate and strategic approach to software design, with a focus on modeling real-world domains and defining a clear separation of responsibilities between different parts of the system. By following DDD principles, the aim is to avoid creating a “big ball of mud” and instead build a well-structured and maintainable software architecture.