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When the game's interface exists inside the game world itself, not floating on your screen like a magic overlay.
Diegetic UI means the characters in the game can see the same interface elements the player sees. Dead Space's health bar is a glowing spine indicator on Isaac's suit. Metroid Prime's HUD is Samus's actual visor, complete with condensation and rain droplets. Far Cry 2's map is a physical object the character holds while walking. This approach sacrifices the clarity of traditional HUDs for dramatically increased immersion. The player isn't reading a game interface; they're looking at the same tools and indicators their character uses.
Example
Dead Space (2008) eliminated the traditional HUD entirely. Health is displayed on Isaac's spine, ammo count is a holographic display on the weapon, and the inventory is a holographic projection Isaac opens in real-time, meaning enemies can still attack while you're managing items. This single design decision defined the game's identity more than almost any other feature.
Why it matters
Diegetic UI represents a philosophy about how much games should break the fourth wall. As VR gaming grows and immersion becomes increasingly valued, diegetic design is moving from a novelty to an expectation. It also forces designers to think creatively about information delivery.
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