Which aspect of testing focuses on how well a product performs under varying conditions?

Non-functional testing is the aspect of testing that focuses on how well a product performs under varying conditions. This type of testing evaluates attributes such as performance, reliability, security, scalability, and maintainability, rather than just whether the software functions correctly (which is the domain of functional testing).

By examining how a system behaves with factors like load and stress, non-functional testing provides insights into its capability to handle demand under different situations, which is crucial for ensuring that a product meets necessary quality standards in real-world usage. It is particularly important for applications expected to function under heavy user loads or those that must meet specific performance benchmarks.

In contrast, functional testing is concerned with whether the features of the software work as intended without focusing on the performance metrics associated with those functionalities. Usability testing examines how user-friendly the software is, and regression testing is aimed at verifying that recent changes have not adversely affected existing functionalities. Each of these areas plays an important role in a comprehensive testing strategy, but non-functional testing specifically targets performance-related criteria.