A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in myelin.
Nerves are part of the peripheral nervous system. Afferent nerves convey sensory signals to the brain, for example from skin or organs, while efferent nerves conduct stimulatory signals from the brain to the muscles and glands.
These signals, sometimes called nerve impulses, are also known as action potentials: Rapidly traveling electrical waves, which begin typically in the cell body of a neuron and propagate rapidly down the axon to its tip or "terminus."
Nerves may contain fibers that all serve the same purpose; for example motor nerves, the axons of which all terminate on muscle fibers and stimulate contraction. Or they be mixed nerves.
Nerve can also refer to a cocky form of self-confidence. For example, "He has a lot of nerve coming to his ex-wife's wedding!" In American English, this use of nerve is synonymous with balls, and in Spanglish (Spanish-influenced English) cojones may be used.