Artistic elements experienced as part of the world in which they occur. In a movie it is sound heard by the characters.
My introduction to this word came about in a moment of synchronicity. Part of the “sound design” for the audiobook of “The Grifter’s Bible” includes a plethora of in-world sound references and I had just recorded one of these instances (“…the sound of a horsehair bow being drawn across high-tension lines.” when I was reading something on The Quietus that used the term diegetic. I can’t remember what though.