A legacy codebase refers to an existing codebase that is old or outdated by current standards. It often has been in use for a considerable amount of time and might have been built using older technologies, patterns, or practices that are no longer in vogue.
A monorepo (short for "monolithic repository") is a software development strategy where multiple projects, often related or interconnected, are stored in a single version control repository. This contrasts with a "polyrepo" approach, where each project or service has its own separate repository.