Linguists are now agreed that the vast majority of words in every language are entirely arbitrary, but a small class of signifiers somehow mimic their signifieds. Examples include "thud," "screech," "bark" (the sound a dog makes, not tree-skin), "pop," and so on. (Of course these words vary from language to language, so the mimicry isn't perfect.) Sometimes poetry can use words which, through their very sound, suggest their subject.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.