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The difference between sounds that exist inside the game world and sounds that only the player can hear.
Diegetic sound originates from within the game world -- a radio playing music that characters can also hear, footsteps on gravel, an NPC talking. Non-diegetic sound exists outside the game world -- the orchestral score, UI notification bleeps, the narrator's voice. The distinction matters because diegetic sounds anchor the player in the world while non-diegetic sounds guide their emotions from the outside. Some of the most clever audio design plays with the boundary: a non-diegetic soundtrack might become diegetic when the camera reveals it's actually coming from a car radio, or a character might react to a sound that was previously non-diegetic, breaking the fourth wall.
Example
In Doom 2016, Mick Gordon's heavy metal soundtrack is non-diegetic -- the demons can't hear it. But in Death Stranding, the licensed songs that play during walking sequences are ambiguously diegetic, as if Sam is listening to them himself. GTA's car radios are a classic diegetic music system that doubles as a gameplay feature.
Why it matters
Understanding this distinction helps you analyze why certain audio choices feel immersive or jarring. Games that deliberately blur the line between diegetic and non-diegetic sound create some of the most memorable and unsettling audio experiences in the medium.
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