root cause

This term is primarily used in professional, technical, and managerial contexts, such as engineering, medicine, and quality assurance. It carries a strong connotation of systemic analysis, implying that surface-level symptoms have been bypassed to find the actual origin of a failure. Using this phrase suggests a commitment to a permanent solution rather than a temporary fix.

In a corporate or industrial setting, the term is often associated with "Root Cause Analysis" (RCA), a formal process of problem-solving. While "reason" or "cause" are general terms, "root cause" specifically denotes the deepest level of causality in a chain of events.