What is Root Cause?

Legal Definition
A root cause is an initiating cause of either a condition or a causal chain that 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 improve performance or prevent an undesirable outcome.

In plain English a "root cause" is a "cause" (harmful factor) that is "root" (deep, basic, fundamental, underlying, initial 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."

Fantin (2014) describes the root cause as the result of the drill down analysis required to discover which is the process that is failing, defining it as "MIN Process" (meaning a process that is Missing, Incomplete or Not followed)

About Root-Cause discovery, see Root cause analysis.
-- Wikipedia
Legal Definition
The main cause for faults or failures even if more causes are evident that began with the initial cause.