Test-driven development (TDD) is a software development approach that involves writing automated tests before writing the code for a new feature or functionality. The idea behind TDD is to use the tests to drive the development process, by defining the requirements for the code and verifying that it functions correctly.

The TDD process typically follows the following steps:

1. Write a test: The developer writes an automated test that defines the requirements for the new feature or functionality.
2. Run the test: The test is run to verify that it fails, as the corresponding code has not yet been written.
3. Write the code: The developer writes the code to implement the new feature or functionality.
4. Run the tests: The tests are run again to verify that the code meets the requirements and functions correctly.
5. Refactor the code: If necessary, the developer may refactor the code to improve its design or maintainability.

TDD is designed to improve the quality and reliability of software, by ensuring that new code is thoroughly tested before it is deployed. It is also meant to encourage the development of modular, testable code, and to facilitate continuous integration and delivery.