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1.
A string of numbers or characters that often come with computer games or software. The user is required to input the CD key to install the product or play online.

A CD-key is verified by being plugged into a mathematical formula and checking to see if it returns the proper value. For example, a very simple CD-key system would check to make sure that all the numbers in the CD-key add up to a specified number. In addition to this, many games and software products require you to verify your CD-key over the internet. However, this doesn't stop people from creating "keygens" or programs that automatically generated valid CD-keys.
When the cheater got caught, they banned his CD-key and now he can't play online anymore.
by Gazuga May 11, 2004
 
2.
The number found on the cover of a CD case. Used to idenitfy the user.
I need a CD key
Not today dude
by sadfjosadj May 09, 2004