A nerve is a group of fibers that send sensation or physical feeling to the brain. Back pain can sometimes be caused by a damaged or pinched nerve.
Your body depends on your nerves for sensing pain, heat, and cold — not to mention making it possible for you to move your muscles. You can also use the word nerve to mean bravery or daring: "She didn't know if she'd have the nerve to skydive when she was finally up in the plane." In the 1500s, to nerve was "to ornament with threads." All of these come from a Latin root, nervus, "sinew, tendon, cord, or bowstring."