Software entities that define ("bootstrap") their own creation, language, UI, editor, etc. in a self-evident way (rather than depending on other things like an IDE, compiler, editors, etc.). It is / includes "everything you need".
The code / interpreter of the system are "written in itself", and can thus self-modify ("self" can mean "user")
Surprising that this is not already common, since the tool for making & editing software is software