Diegetic (from film theory) describes elements that exist within the story or game world. In game UI, a diegetic element is one the player character could plausibly perceive — a health bar painted on a character’s armour, ammo displayed on a weapon’s side panel, a map existing as a physical object in-world. Non-diegetic elements (traditional HUDs, pause menus) float outside the game world as pure interface (course lecture series, see source-player-psychology-lectures).
The diegetic/non-diegetic distinction is a design axis, not a quality judgment — both have appropriate uses. Immersive sims often favour diegetic UI to maintain presence; action games often favour non-diegetic for clarity.
In Fagerholt and Lorentzon’s (2009) interface model, belonging to the fiction is not enough on its own to make an element diegetic. A second question, whether it sits in the game’s 3D world, separates diegetic elements (fiction and world) from meta elements (fiction, drawn on screen), and non-diegetic elements (neither) from spatial elements (world, player only) (course diagram on interface types, see source-four-types-of-game-interface). See game-interface-types for the full model.
The thesis explains this choice. The authors first used diegesis itself as one axis of their grid, but dropped it because it created grey areas, such as Killzone 2’s blood on the screen and Mirror’s Edge’s Runner Vision. They replaced it with a fiction axis (“Is the UI element existing in the fictional game world?”), so that “diegetic” became the area where fiction and 3D space overlap. They also define signifiers as a sub-group of diegetic elements: indirect cues the player has to reason about, such as smoke from a weapon about to break. They warn that a fully diegetic interface “is not automatically more immersive” (Fagerholt and Lorentzon, Beyond the HUD, pp. 48–52, 75, see source-beyond-the-hud).
Related: game-interface-types, ui-design, presence-and-immersion, game-feel, narrative-design, source-beyond-the-hud