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Thanks to Craig Buchek for the content of this page.
Security theater is any measure that gives the appearance of providing security, while doing very little to actually enhance security. By giving the appearance of security, people feel more secure, even when they are not.
The prime example of security theater seems to be airport security. Making people take their shoes off and limiting liquids is very unlikely to prevent future attacks. But going through the process makes people think that the authorities are doing something \1nd therefore things are more secure and under control.
The down side \1f security theater is it costs money which would be far better spent on things which really would improve security, such as enhanced training regimens for TSA staff.