The origin or source of the problem.
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.
The term root cause has been used in professional journals as early as 1905, but the lack of a widely accepted definition after all this time indicates that there are significantly different interpretations of exactly what constitutes a root cause.
The two biggest differences in viewpoint regard the possibility of an outcome having more than one root cause