"TDD" refers to Test-Driven Development, a software development practice where tests are written before the actual code implementation. In TDD, developers first define the expected behavior of the code through test cases, then write the minimal code necessary to pass those tests. This iterative cycle of testing and development encourages cleaner, more reliable code and facilitates better design and architecture. TDD promotes confidence in code quality as tests serve as a safety net for future refactoring, making it easier to adapt and maintain software over time.