Test-driven development (TDD) is a software development approach in which tests are written before the actual code is developed. The aim of TDD is to ensure that each piece of code is fully tested and meets the requirements of the business. TDD involves writing a failing test first, writing just enough code to make it pass, and then refactoring the code to improve its design while still ensuring that all tests pass.

TDD is an important practice in Domain-Driven Design (DDD) because it helps ensure that the code developed is in line with the domain model and the domain rules. By writing tests first, the developer is forced to think about the domain model and the domain rules before writing any code. The tests become a way to specify the behavior of the system and help keep the focus on the business requirements. As the code is developed and refactored, the tests ensure that the system remains aligned with the domain model and the business requirements.