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

The focus of non-functional testing is on evaluating how well a product performs under varying conditions, which includes aspects such as performance, reliability, scalability, and security. This type of testing assesses the quality attributes of the system rather than its specific functionalities or behaviors.

For instance, non-functional testing may measure how a web application handles a large number of users simultaneously, how quickly it responds under different network speeds, or how well it maintains performance when loaded with significant data. These metrics are crucial for ensuring that the product not only works but works effectively under real-world conditions that users might encounter.

In contrast, functional testing targets the specific features and behaviors of the application to ensure they meet the requirements, usability testing assesses the user experience and satisfaction, and regression testing focuses on verifying that existing functionalities remain intact after changes are made to the code. Each of these areas serves an important purpose, but they do not specifically address how well a product performs when faced with changes in environmental factors, which is the core concern of non-functional testing.