A test plan in software testing is a comprehensive document that acts as a blueprint for the entire testing process. It defines the testing approach, identifies the features to be tested, and specifies the test schedule, resources, and tools required. The test plan also includes risk analysis, entry and exit criteria, and roles of team members. By providing a structured framework, it ensures thorough testing of the software, uncovering bugs and issues before release. An effective test plan aligns the testing efforts with project goals, improving the overall quality and reliability of the final product.