The means by which an artist creates a recognizable, pleasing representation of something in the real world. Aristotle refers to "persons who, by conscious art or mere habit, imitate and represent various objects through the medium of color and form, or again by the voice" (Poetics I ¶4). Children who imitate the behavior of adults, dancers who imitate the behavior of animals, and actors who imitate the behavior of other people are all drawing upon their mimetic talents. Mimesis involves "showing" in contrast to diegesis, which is "telling."
A work of interactive fiction is said to commit a "sin against mimesis" (after an influential 1996 Usenet posting by Roger Giner-Sorolla, "Crimes Against Mimesis") when flaws in the game design call too much attention to the interface. Examples may include a scenery object (such as the sky or the grass) that is mentioned in a room description, but is not implemented; or, when an NPC's default responses to a player's actions are glaring non sequiturs (such as early versions of Suspect (Dave Lebling; 1984; Z-code), which provide the default response, "Michael doesn't seem to be interested in that," in response to "show corpse to Michael").
Like any contract, the mimesis agreement is subject to negotiation (or dissolution). A game that lacks mimesis is not necessarily a failure, and a game that contains nothing but mimesis is not necessarily enjoyable. In some cases, the game designer may intentinally poke holes in mimesis; Nick Montfort's Ad Verbum (2000, Z-code) makes direct references to the PC being a seasoned text adventure gamer, and the narrator of Adventure occasionally poses questions directly to the player. (Montfort himself explains these and similar cases by relating them to metalepsis, describing IF as diegetic rather than memetic.)
In essence, mimesis is interactive fiction's answer to the fourth wall of theatre and film. Like the fourth wall, it is a contract entered into between the player and author. The author agrees to provide a coherent world in which the player can lose themselves, and the player, in turn, willfully suspends disbelief.
A sin against mimesis can, therefore, come from either author or player. It occurs when one or the other does not perform their "job" properly. Just as the player should be able to expect a logical and world-consistent response to "hit troll with sword," it is not necessarily the responsibility of the author to come up with a creative response to "eat sword." For when the player does this, they are abandoning the context of the game—they are metagaming.
See also immersion.