A "Test Double" is a general term used in software testing to describe a replacement for a real component in a system. It allows developers to simulate the behavior of complex or unavailable components, making it easier to test specific parts of the code without relying on the actual implementation. Common types of test doubles include stubs, mocks, and fakes. Using test doubles helps improve the efficiency and reliability of tests. They can isolate the unit being tested, ensuring that tests focus on the functionality of that unit without interference from other components. This leads to faster feedback and more accurate identification of issues in the code.