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Definition: the 5 Whys is an iterative root cause analysis technique which works by repetitive asking 'Why?', exploring the cause-and-effect relationships underlying a particular problem.
The primary goal of the technique is to determine the root cause of a defect or problem. Typically this should be a broken process or an alterable behavior.
Although the number of 5 iterations has proven sufficient to arrive at the root cause, this should not be taken as a fixed rule; sometimes a 6th or 7th question may be needed.
The tool is used - often in combination with an Ishikawa Diagram - within Toyota Production System, Kaizen, Lean Manufacturing and Six Sigma projects.