History of vi
The vi editor was developed at the University of California, Berkeley, as part of Berkeley UNIX (BSD), and is included as a standard utility in SVR4. Before vi was developed, the standard UNIX system editor was ed. The ed editor was line-oriented, which made it difficult to see the context of your editing. Then ex came along--ex was a superset of ed. The most notable advantage that ex had over ed was a display editing facility that allowed users to work with a full screen of text instead of with only a line at a time. While you were using ex, you could use the display editing facility by giving ex the command vi (for visual mode). People used the display editing facility of ex so extensively that the developers of ex made it possible to start the editor so that you were using the display editing facility at once, without having to start ex and give the vi command. Appropriately, they named the new facility vi.
---UNIX System V, A Practical Guide 3rd Ed.