onomatopoeia (ˌɒnəˌmætəˈpiːə) — n
1. the formation of words whose sound is imitative of the sound of the noise or action designated, such as hiss, buzz, and bang
2. the use of such words for poetic or rhetorical effect
"Zip" is an example of onomatopoeia because it sounds like what it is. When you zip up a zipper the sound the zipper makes sounds like a zipper. Here are other onomatopoeia words:
Boom, bang, slash, slurp,
gurgle, meow,and woof