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A root cause is an initiating cause of a causal chain which leads to an outcome or effect of interest. Commonly, root cause is used to describe the depth in the causal chain where an intervention could reasonably be implemented to change performance and prevent an undesirable outcome.
In plain English a "root cause" is a "cause" (harmful factor) that is "root" (deep, basic, fundamental, underlying or the like).
The term root cause has been used in professional journals as early as 1905.
Paradies (2005) has defined a root cause as follows: "The most basic cause (or causes) that can reasonably be identified that management has control to fix and, when fixed, will prevent (or significantly reduce the likelihood of) the problem’s recurrence." 
- Forensic engineering
- Proximate and ultimate causation
- Root cause analysis
- RPR Problem Diagnosis