Strictly speaking, the formation or use of words which imitate sounds, like whispering, clang, and sizzle, but the term is generally expanded to refer to any word whose sound is suggestive of its meaning whether by imitation or through cultural inference.
Sidelight: The use of onomatopoeia is common to all types of linguistic expression, but because sound plays such an important role in poetry, it provides another subtle weapon in the poetic arsenal for the transfer of sense impressions through imagery, such as Keats' "the murmurous haunt of flies on summer eves," in "Ode to a Nightingale."
Sidelight: Though impossible to prove, some philologists (linguistic scientists) believe that all language originated through the onomatopoeic formation of words.
(See also Mimesis, Phonetic Symbolism)

This definition is in context to Poetry. See more contextual defintions for Onomatopoeia (Ahn-uh-mah-tuh-pee-uh).