What is test oracle?

 A test oracle is a mechanism or principle that can be used to determine whether the observed behavior or output of a system under test is correct or not1. A test oracle can be based on various sources of expected results, such as specifications, user expectations, previous versions, comparable systems, etc2. References: ISTQB Certified Tester Foundation Level (CTFL) v4.0 Syllabus, Section 1.2.1, Page 91; ISTQB Glossary of Testing Terms, Version 4.0, Page 332.