What characteristic defines unit testing in software development?

Unit testing is defined by its focus on verifying individual components of the software. This form of testing targets the smallest testable parts of an application, typically functions or methods, to ensure they perform as expected. By isolating each component, developers can catch bugs and issues early in the development process, which contributes to overall software quality and reliability. This testing is usually automated, allowing for quick execution and consistent testing over time.

The other options describe different aspects of software testing. For instance, testing the interaction between components is characteristic of integration testing, assessing overall system performance pertains to performance testing, and manual testing processes refer to a broader spectrum of quality assurance practices rather than the specific automated and focused nature of unit testing.