A sound which can be paired with what is happening in a film or on TV, and is either visible, or is relevant to the action.
Non-diegetic means a sound that cannot be seen on screen, nor has any direct relevance.
Diegetic:- A war film with people firing guns, and some of the shot sounds can be seen on screen, while others cant, but remain relevant to the action.
Non-Diegetic:- A war film with the sound of a cymbal crashing shortly before someone firing. The sound is not relevant to what is happening, nor is implied to be happening on or off screen.