alt Hacker NewsA butterfly valve is a category of thing. The corresponding word for emacs would be "editor." That is entirely descriptive: an editor edits.
Picking a specific butterfly valve randomly from an internet search, I find one called the FNW FNWHPA1LSTG24.
Product types and categories get generic names, specific products often get weird names. It's true in just about every field.
Someone was probably the first to call their valve a butterfly valve.
Emacs can also be taken to be a category of editors. There are multiple emacs-derived editors.