1. /V-I/, not /vi:/ and never /siks/ n. [from `Visual Interface'] A screen editor crufted together by Bill Joy for an early BSD release. Became the de facto standard Unix editor and a nearly undisputed hacker favorite outside of MIT until the rise of EMACS after about 1984. Tends to frustrate new users no end, as it will neither take commands while expecting input text nor vice versa, and the default setup on older versions provides no indication of which mode the editor is in (years ago, a correspondent reported that he has often heard the editor's name pronounced /vi:l/; there is now a vi clone named `vile'). Nevertheless vi (and variants such as vim and elvis) is still widely used (about half the respondents in a 1991 Usenet poll preferred it), and even EMACS fans often resort to it as a mail editor and for small editing jobs (mainly because it starts up faster than the bulkier versions of EMACS). See holy wars. From Jargon Dictionary

2. On UNIX, the vi program is a small text editor that can be run from the command-line. It can even be run in ed/ex mode that runs in line-mode rather than full-screen mode. Since vi is included on every UNIX system, this is the one program that all hackers learn to use. (More advanced editors like emacs may not be installed on a system that a hacker breaks into, leaving them out of luck if they don't know vi). From Hacking-Lexicon