No, not onomatopoeia, which is more of a process. The word I want is the specific term for a word like "boom," "thump," or "poof" -- words which are iconic representations of specific sounds or actions and do not necessarily correspond accurately to the sound or action they are meant to represent. I know this word exists; I just forgot to write it down when I encountered it, and now I've forgotten where.

Welcome aBoard, fibitz. I don't know the word you're looking for, but I put your description into Onelook's Reverse Dictionary; and even though I thought it would be too long for Onelook to cope with, it gave me a nice list. If you want to look through it, maybe you'll recognize the word you're looking for, or perhaps can try a different description.
web page

http://dictionary.reference.com/search?q=onomatopoeia&r=66

Thus you can use it as the word itself
Else go here:

http://thesaurus.reference.com/browse/onomatopoeia