A root cause, at the most basic level, is a fundamental reason for the occurrence of a problem or event. The appellation "root" is meant to differentiate a root cause from a more immediate or proximate cause; in other words, it can be considered an ultimate cause.
There are many definitions for root cause in common use. They are often custom-tailored to a particular user or industry, Root Cause Analysis vendor or consultant, or as necessary for a specific problem domain. This wiki page will not attempt to provide an exhaustive list of such definitions, or provide one intended to subsume them all. It will, however, describe an early concept of root cause that still aligns well with common contemporary thinking, and then provide a few of the more common definitions in circulation (as of 2014).
One early conception of root cause in the English language was penned by Thomas Hobbes in the late 1640s. In the following passage, Hobbes basically explains the entire concept of root cause (remote causes), why someone might want to practice root cause analysis, outlines a basic technique for finding root causes, and even identifies what he believed to be the penultimate root cause.
Ignorance of remote causes disposeth men to attribute all events to the causes immediate and instrumental: for these are all the causes they perceive...
Anxiety for the future time disposeth men to inquire into the causes of things: because the knowledge of them maketh men the better able to order the present to their best advantage.
Curiosity, or love of the knowledge of causes, draws a man from consideration of the effect to seek the cause; and again, the cause of that cause; till of necessity he must come to this thought at last, that there is some cause whereof there is no former cause, but is eternal; which is it men call God.
Thomas Hobbes, Leviathan, Chapter XI (1651)
One could, in theory, read and apply only that one little excerpt from a 17th century book and begin investigating problems... and probably be more successful at solving them than the majority of people that base their corrective actions solely on "causes immediate and instrumental". (Wiki editor's note: How is Hobbes' suggested method different from 5 Whys?!) Of course, being the modern humans that we are, we must develop and use new definitions and tools. Here are some definitions of root cause that are fairly representative of modern thinking.
Root cause is that most basic reason for an undesirable condition or problem which, if eliminated or corrected, would have prevented it from existing or occurring.
Wilson, Paul F., Dell, Larry D., and Anderson, Gaylord F., Root Cause Analysis,
ASQ Quality Press (1993).
An original cause, through a chain of cause and effect, of an undesirable effect (UDE).
Dettmer, William H., Goldratt's Theory of Constraints,
ASQ Quality Press (1997).
Any cause in the cause continuum that is acted upon by a solution such that the problem does not recur.
Gano, Dean L., Apollo Root Cause Analysis (2nd ed),
A Root Cause is the absence of a best practice or the failure to apply knowledge that would have prevented the problem.
Paradies, Mark and Unger, Linda, TapRooT,
System Improvements (2008).
This wiki topic is likely to be expanded greatly in the future. However, as one final thought about the topic of root cause, consider the following.
by Bill Wilson