"[...] a fault is a potential failure, which can cause an incorrect result (failure) in the future, or may have already caused one. The failure may not have observable consequences, however, and so may not be reported as a problem." (Richard D Stutzke, "Estimating Software-Intensive Systems: Projects, Products, and Processes", 2005)
"1. Deviation of the component or system from its expected delivery, service, or result. (For a test result, the observed result and the expected [specified or predicted] result do not match.) 2. Result of a fault that, during test execution, shows an externally observable wrong result. 3. Behavior of a test object that does not conform to a specified functionality that should be suitable for its use." (Tilo Linz et al, "Software Testing Foundations, 4th Ed", 2014)
"The condition of not achieving the desired end results." (Ken Sylvester, "Negotiating in the Leadership Zone", 2015)
"A system failure is an event that occurs when the delivered service deviates from correct service (implementing system function). A system may fail either because it does not comply with the specification or because the specification did not adequately describe its function." (O Sami Saydjari, "Engineering Trustworthy Systems: Get Cybersecurity Design Right the First Time", 2018)
"Deviation of the component or system from its expected delivery, service or result." (Software Quality Assurance)
"Loss of ability to operate to specification, or to deliver required output" (ITIL)
"The inability of a system or component to perform its required functions within specified performance requirements. The fault tolerance discipline distinguishes between a human action (a mistake), its manifestation (a hardware or software fault), the result of the fault (a failure), and the amount by which the results are incorrect (the error)." (IEEE Std 610.12-1990)