What is onomatopoeia?
By most definitions, onomatopoeia means the use of words that imitate sound. It can also refer to the property of such words. According Wikipedia, onomatopoeia can be a countable noun, so kaboom would be an onomatopoeia, and the plural would be onomatopoeias. The adjective onomatopoeic is synonymous with echoic or imitative. The word comes from Greek and roughly means "name-making", which makes sense because the sound makes the meaning. Onomatopoeia is found everywhere: in poetry, children's books, comics, literature, advertisement, art, in everyday conversation and on the web.
Besides the obvious examples, there are many words that originate from words that were imitative, but are not clearly imitative in their current form. For example, according to Etymonline, buffoon comes from the Italian word buffare, meaning "to puff out the cheeks", which was a comic gesture in the 16th century and is therefore of imitative origin. Nowadays, buffoon still means "a ridiculous but amusing person; a clown", but we don't associate it with puffing out the cheeks anymore, nor do we find puffing out the cheeks all that hilarious.
Onomatopoeia for the same thing can vary quite a bit. For example, in Aristophanes' comic play The Frogs (Ancient Greek), the frogs say brekekekex koax koax whereas nowadays in English we use ribbit, although some say they actually imitate a different species of frog! Spelling can also vary quite a bit. Of interest here is a webpage that shows an analysis of the many different spellings of Aargh, called The Aargh Page.