Literally, imitation or realistic representation -- but its poetic significance is more specific: it refers to the combination of sound in phonetic symbolism and onomatopoeia (sound suggestion) with the connotative, symbolic, and synesthetic effects of the words themselves and their syntactic arrangement to resemble, reinforce, shape, and temper their lexical sense in a manner that mirrors the meaning. In An Essay on Criticism, Pope simplified with the precept, "the sound must seem an echo to the sense." He wrote the following couplet to illustrate:
Soft is the strain when Zephyr gently blows,
And the smooth stream in smoother numbers flows.
(See also Ekphrasis, Sound Devices)

This definition is in context to Poetry. See more contextual defintions for Mimesis (Mih-mee-sis).