CLIAn acronym for Command Line Interface, a kind of user interface.
CLIs use a simple paradigm of issuing commands to the system using words. CLIs are very efficient, but their commands are rarely visible. A CLI with good, consistent names and thorough documentation would be somewhat easy to learn, and fast.
The classic example is the Unix command line. This typically operates under a shell (e.g. bash, ksh, and the author's favorite, zsh) which is a simple but almost always turing equivalent language. Unfortunately, the Unix naming convention is something along the lines of "if the name is understandable, relevant to whatever it represents, or consistent in any way with the rest of the system, rename it", making using the Unix command line hell to the uninitiated. Otherwise it is very efficient.
The big problem with CLIs is the shortage or, sometimes, total absence of feedback. A better paradigm, which subsume CLIs, are TUIs.
A less-common use of the abbreviation CLI refers to the Common Language Infrastructure, which is the proposed specification upon which the Microsoft .NET CLR is based. The CLI has been submitted to the ECMA for independent global standardization.