Definition: configuration file
A file that contains data about a specific user, program, computer or file. Used for myriad reasons, configuration files are generally read at startup by the operating system as well as by applications in order to customize the environment for the user. In the Windows world, especially the earlier versions of Windows, .INI (initialization) files have been used. In Java, configuration data are stored in properties files. The Registry is also a storehouse of configuration data, which is widely used.
Configuration files typically store data in a key=value structure, which means each item of data (the key) has a name, and its value is its contents. For example, defaultFont=Arial.