It’s a text editor. It all began with the ed editor, which is very simple and does one thing, it edits files. Then someone extended it into the ex editor. Then someone added a new feature: being able to visually see the file you’re editing, which became vi, the visual editor. Then someone improved that, into vim. What began as an editor where you needed to be fluent in regular expressions but otherwise was simple, is now a very complex editor, moving the functionality of the old UNIX tools into the editor itself.