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. Huckleberry Finn would be an example. Note that if the characters are below our level, it's in the ironic style.
See also: Mimesis, Modes of Literature.