What's the definition of a legacy system?

Today I found this funny quote in http://www.texas400.com/

I created an stupid poster for it :)

What's the definition of a legacy system ? One that works!

Updated: Of cuorse, I do not mean that "Legacy systems are better". I see in this quote a ironic fact: you spend time and efford on a system and when it finally works properly... it is legacy and should be replaced :)