Simple Definition:

Onomatopoeia is when words imitate the sounds they represent. It is a way to describe things using words that sound like the thing being described. For example, words like “buzz,” “hiss,” and “woof” are onomatopoeic because they sound like the noises they represent.

Very Simple Definition:

Onomatopoeia is when words sound like the things they describe. For example, “boom” is the sound of an explosion and “meow” is the sound a cat makes.