Diegesis is the process where role-playing game participants share things or events into play through representational discourse (often speech), including many non-visual or indirectly related items, such as a character's thoughts, beyond-play elements, events happening elsewhere, conflicts prior to the game or et cetera. This has been commonly used in traditional tabletop role-playing games? going all the way back to Dungeons & Dragons?

Diegesis in Contrast to Mimesis

Diegesis is often contrasted with mimesis?, which is an approach that is not representative but rather is demonstrative (for example; by gesturing, mimicking or expressing what is shared rather than describing it).

Diegesis in Narratology

Diegesis is also the sum narrative? in fiction and drama, the telling of the story, speaking directly to the audience, for example, the invisible narrator commenting on the action or the characters.

Diegesis in Film

In film, diegesis is the narrative? including the whole story plus everything that is and isn't actually shown (events before the present; people mentioned; or events that happen elsewhere).

Elements of a film can be "diegetic" or "non-diegetic." If a character is playing a piano, the resulting sound is "diegetic." For example, voice-over narration or the score of a film playing in the background that cannot be heard by the film's characters is non-diegetic or, more accurately, extra-diegetic.

Diegesis in LARP Worldwide

The term has also been borrowed from film by Nordic LARP role-playing theory: "Diegesis is what is true within the game." This is discussed extensively in Beyond Role and Play. And is essentially the same as The Forge Forum term Shared Imaginary Space.

References:

• Diegesis on Wikipedia
• The Meilahti School
• As Larp Grows Up
• Beyond Role and Play
• The Forge Forum