mimesis (mi-me'sis, mi-) n. 1. The imitation or representation of aspects of the sensible world, especially human actions, in literature and art.
[the following is stolen from a post to raif by Adam Cadre]
In brief, there are generally two different things people on this group use the word "mimesis" to refer to:(1) The extent to which the player feels like she's experiencing what the game tells her she is, rather than experiencing the sensation of typing on a keyboard and watching words scroll by on a screen;
(2) The extent to which stuff in the game seems to work the way things work in real life, or at least the extent to which it maintains a degree of internal consistency.
The idea of mimesis with regard to IF was started by Roger Giner-Sorolla in his "Crimes Against Mimesis" postings some time back on raif. You can find a copy of it at <http://bang.dhs.org/if/library/design/mimesis.html>
More recently, Adam "Bruce" Thornton wrote a very funny in-joke game called "Sins Against Mimesis".