vi is a ubiquitous text editor found on just about every UNIX operating system you’re likely to run across. It has been copied and cloned many times, and now exists in much improved form under the guise of Vim, the most successful and powerful of the vi clones.

It’s distinguishing feature is that it is a modal editor. Perhaps in the days when vi was first invented that wasn’t such a special thing at all; but in modern times (perhaps as much as two or three decades) non-modal editors such as Emacs and just about every word processor out there today have become so dominant that well over 95% of computer users have probably never seen a modal editor.