Here's a breakdown:

* Nervous system: This is the body's control center, responsible for receiving and processing information and sending signals to different parts of the body.

* Myelinated fibers: These are nerve fibers (axons) that are covered in a fatty substance called myelin. Myelin acts as an insulator, allowing nerve impulses to travel faster and more efficiently.

* White matter: This is the part of the nervous system that is composed of these myelinated fibers. The myelin gives it its white color.

Therefore, white matter refers specifically to the myelinated fibers within the central nervous system (brain and spinal cord) and peripheral nervous system (nerves that connect the CNS to the rest of the body).