An authentication key is a code that guarantees the installation program (as part of software) that the installer or user is the right owner of the said software. It differs from a password that a key is used just once (during installing) and a password is required each time you want to access or use the software.
Some software is limited in use (possibility or time) and the limitations are lifted when the key is entered. This software is often free, but you have to pay for the key.
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