Onomatopoeia

Definition: words which mean what they sound like

Example:
Bang! The bullet goes through the man's head, splashing his brains everywhere.

Example of opening sentence - by Blake Stok, a.k.a Mr Blake

Significance: Sometimes just describing the sound isn't enough. The author describe the sound of a the gun shot (without telling the reader it's the sound of a gun shot)as loud and scary, but the reader might think of the sound of an earthquake since his he think of loud and scary is the sound of an earthquake, not a gun shot.