Diegetic events are those experienced by both the characters within a piece and the audience, while non-diegetic elements of a story make up the “fourth wall” separating the characters from the audience.
I encountered this word via this post on the Unsung blog, where it is repurposed outside of fiction storytelling to indicate a user interface that can manipulate itself; the author says:
inexplicably on macOS the floating thumbnail is diegetic, meaning it itself can be screenshotted.
A few days later, the word resurfaced on the same blog.