Unit testing is a software testing method that focuses on individual units or components of a software system. A unit can be a function, method, module, object, or other entity in an applications source code. The purpose of unit testing is to validate that each unit of the software works as intended and to ensure that new code does not break existing functionality. Unit tests are typically automated tests written and run by software developers to ensure that a section of an application (known as the "unit") meets its design and behaves as intended.

Unit testing is an important step in the development process, as it can detect early flaws in code which may be more difficult to find in later testing stages. By testing individual components in isolation, developers can quickly identify and address issues, leading to faster iteration and release cycles. Unit testing also enables more frequent releases in software development.

Unit tests tend to be easiest to write and run when they focus on small, isolated units of code. Developers can perform unit tests manually or automatically, with automated testing being the more common method. Automated approaches commonly use a testing framework to develop test cases. Effective unit testing typically runs each test case in an isolated manner, with "stubs" or "mocks" used to simulate external dependencies.

Overall, unit testing is an important part of the software development process that helps ensure the stability and reliability of the code, allowing organizations to deliver timely updates and enhancements to users.