A license key - also called product key or key code - is a number, letter code that must be entered during the installation or the first start of a software in order to activate it. It usually consists of a sequence of numbers and letters divided into several blocks. By using a license key, program files of a certain software (e.g. operating system or office software) can be copied, but the software cannot be used without the key. This is to prevent the illegal use of unlicensed software.
In the past, the license key was always attached to a physical data carrier (such as a CD-ROM or DVD) in the form of a CD key. In the meantime, a license key is also used for software that is distributed via ESD without a data carrier. Product keys belong to the so-called "digital goods".