System testing is:

Concerned with the behavior of a whole system/product as defined by the scope of a development project. System testing is a level of testing that evaluates the system as a whole against its requirements and specifications. System testing is performed after integration testing and before acceptance testing. System testing covers functional and non-functional aspects of the system, such as usability, performance, reliability, security, etc. A detailed explanation of system testing can be found in A Self-Study Guide For The ISTQB Foundation Exam Certified Tester Foundation Level (CTFL) 2018 Syllabus, pages 75-782.