The use of words that supposedly mimic their meaning in their sound. Such devices bring out the full flavor of words.
Tennyson makes us feel the heaviness of a drowsy summer day by using a series of "in" sounds in the wonderfully weighted lines:
"The moan of doves in immemorial elms,
And murmuring of innumerable bees."
Or
Edgar A. Poe lets us hear the different kinds of sounds made by different types of bells in his famous poem "The Bells."