A usability test is defined in the UXQB curriculum as follows:

"A usability evaluation in which representative users perform specific tasks with the interactive system so that usability problems can be identified and analyzed or effectiveness, efficiency, and satisfaction can be measured."

What is the process of a usability test?
A usability test consists of three phases. First comes the planning and this includes writing the usability test plan, writing the usability test script and recruiting appropriate usability test participants. Second comes the execution of the usability test sessions and in the last phase the usability findings are communicated and the usability test report is written.

Why are usability tests conducted?
Usability tests are conducted mainly for the following reasons, to assess whether the usage requirements have been met, to uncover usability problems so that they can be corrected, or to show stakeholders in a convincing way that their interactive system has serious usability problems that can be found and fixed.

But usability testing is unique in one respect, because it shows what representative users can accomplish with the interactive system. This is exactly why usability tests are important for product development. The company is enthusiastic about the interactive system, but the view from the outside is needed. Preferably from the target group for which the interactive system was developed. This ultimately ensures that the user can use the interactive system properly and intuitively.

When can a usability test be conducted?
A usability test can take place at any time during the human-centered design process, from early analysis to delivery of the interactive system, or even afterwards. A usability test can equally focus on a prototype in the form of paper drawings, or on the interactive system being developed or completed.

Source: UQXB Curriculum CPUX-UT 2020, https://www.phmu.de/blog/warum-usability-testing-wertvoll-ist-und-wie-es-funktioniert/