Diegesis is a term taken from game studies, where it refers explicitly to the narrative game world; its characters, objects and actions which comprise the fiction of the game world. It is contrasted against extra-diegetic or ludic elements of the game such as the control interface, information accessible to the player about their character's health or other statistics.
In roleplaying, the diegetic parts of a game are those accessible to the character, while the ludic parts of the game are those accessible to the player. This distinction allows for broader discussion of gameplay elements on both sides of the IC/OC divide.
Players who prefer high immersion roleplay experiences generally concentrate on maximising diegetic elements and minimising ludic interventions, while players who prefer competitive roleplay experiences and character advancement generally concentrate on maximising fair and accessible ludic interactions.