From Greek mimeisthai (to imitate) + -ikos (-ic).
As Webster says, mimetic means imitative. You can have mimetic actions, mimetic plants, or mimetic dance (mimetic is related to mime). A mimetic play might have more mime than words. You can also have mimetic crystals, meaning just that they happen to look like some other structure.
A 'mimetic word' is synonymous with 'onomatopoeia'.
One hopes that a mimetic writing or dialog mimics real life. The two types of mimetics in the narrative tradition are:
High Mimetic -- in which the central characters are above our own level of power, action, excitement and authority. This is often over dramatic. Check out Odysseus.
Low Mimetic -- the characters are basically just like you or I. Off the top of my head the best I can come up with here on E2 is I don't care about society, it just gets in the way of my individual freedom. /msg me if you have a better example.
(If the characters are below our level, it's in the ironic style.)
See also: Mimesis, Modes of Literature.