The practice of asking why repeatedly whenever a problem is encountered in order to get beyond the obvious symptoms to discover the root cause.
For instance, Taiichi Ohno gives this example about a machine that stopped working (Ohno 1988, p. 17):
1. Why did the machine stop?
There was an overload and the fuse blew.
2. Why was there an overload?
The bearing was not sufficiently lubricated.
3. Why was it not lubricated?
The lubrication pump was not pumping sufficiently.
4. Why was it not pumping sufficiently?
The shaft of the pump was worn and rattling.
5. Why was the shaft worn out?
There was no strainer attached and metal scraps got in.
Without repeatedly asking why, managers would simply replace the fuse or pump and the failure would recur. The specific number five is not the point. Rather it is to keep asking until the root cause is reached and eliminated.