NUnit is an open-source unit testing framework for .NET languages. It is widely used for writing and executing tests in C# among other .NET languages. Influenced by JUnit, NUnit brings the power of unit testing to the .NET framework, facilitating test-driven development (TDD) by allowing developers to write code iteratively and test as they progress. The framework supports data-driven tests, and its rich set of assertions makes it easier to verify code against expected outcomes. NUnit is available under the MIT License, promoting widespread use and modification without extensive licensing restrictions.