Lock Out, Tag Out

A safety procedure used to ensure that a machine is properly shut off and can't unexpectedly energize.

According to OSHA, the following steps must accomplished:

1. Prepare for shutdown;
2. Shut down the machine;
3. Disconnect or isolate the machine from the energy source(s);
4. Apply the lockout or tagout device(s) to the energy-isolating device(s);
5. Release, restrain, or otherwise render safe all potential hazardous stored or residual energy. If a possibility exists for reaccumulation of hazardous energy, regularly verify during the service and maintenance that such energy has not reaccumulated to hazardous levels; and
6. Verify the isolation and deenergization of the machine.

This can also be applied when two or more people are working on the same machine. From Wikipedia:

When two or more people are working on the same or different parts of a larger overall system, there must be multiple holes to lock the device. To expand the number of available holes, the lockout device is secured with a folding scissors clamp that has many pairs of padlock holes capable of keeping it closed. Each worker applies their own padlock to the clamp. The locked-out machinery cannot be activated until all workers have removed their padlocks from the clamp.