Final answer:
The insulating lipid layer that covers some neurons is known as myelin, and it significantly speeds up the transmission of electrical signals within the nervous system.
Step-by-step explanation:
The term that best defines the insulating lipid layer covering some neurons in the brain and spinal cord and most peripheral nerves is myelin. The myelin sheath is this lipid-rich layer of insulation surrounding the axon of a neuron, which is key for the rapid and efficient transmission of electrical signals. It is formed by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). The presence of myelin allows action potentials to jump from one node of Ranvier to the next in a process called saltatory conduction, greatly speeding up neural communication compared to unmyelinated fibers.