Short for Command Line Interface. A software that has no GUI. The user issues commands to the program in the form of successive lines of text. There has been one very popular CLI tool for Drupal for many years: Drush, short for Drupal Shell. Starting with Drupal 8 a second CLI tool has been introduced called Drupal Console. Those CLI tools are used by developers and administrators, ordinary Drupal users will most likely never get in touch with them.