Final answer:
Unit testing specifically refers to testing the functions of a single class to ensure that individual components of the software work as intended. It is distinct from other types of testing like usability or integration testing.
Step-by-step explanation:
An example of unit testing is testing the functions of a single class. Unit testing involves verifying that individual components or units of source code, like functions or classes, work as intended. It is an isolated form of testing, usually performed by developers to ensure that each unit of the software performs correctly.
Other methods mentioned, such as analyzing program properties with a static analyzer or testing system features from a user's perspective, correspond to different types of software testing, like static analysis and acceptance testing respectively. Testing the usability of an interface based on a heuristic checklist would be considered usability testing, and testing the interaction between two classes is more akin to integration testing.