The use of words to imitate the sounds they describe. Words such as buzz and crack are onomatopoetic. The following line from Pope's "Sound and Sense" onomatopoetically imitates in sound what it describes

When Ajax strives some rock's vast weight to throw, The line too labors, and the words move slow.

Most often, however, onomatopoeia refers to words and groups of words, such as Tennyson's description of the "murmur of innumerable bees," which attempts to capture the sound of a swarm of bees buzzing.