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A string of numbers and letters used to identify a software product and make sure that the user is using a valid copy. Sometimes referred to as a CD key.
I had to type in the 25-digit Windows XP product key in TWICE cause I mistyped one letter.

I bet Windows 2099 will require users to enter a 200-digit product key and then re-confirm it.
by computer_guy February 18, 2008
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