From Wiktionary, the free dictionary

Noun

root[6] cause[1] (plural root causes)

(idiomatic) An initiating cause of a chain of events which leads to an outcome or effect of interest.

Verb

root cause (third-person singular simple present root causes, present participle root causing, simple past and past participle root caused)

(transitive) To identify the cause of a chain of events which leads to an outcome or effect of interest.

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