(1) Short for command line interface, a user interface common to MS-DOS computers. The user sees the command line on the monitor and a prompt that is waiting to accept instructions from the user. The user types in the command, the computer acts on that command and then issues a new prompt for the next instruction from the user.
CLI operating systems are becoming less used as GUI operating systems gain in popularity. In a GUI operating system, such as Windows, the user responds to graphic images on the screen instead of typing in commands in response to a prompt.
(2) Short for Common Language Infrastructure.