The elbow was the joint between the upper arm and the forearm.
In 2268, Doctor Leonard McCoy, as he performed brain surgery on Spock, stimulated the nerve endings and observed the physical reaction. One of the nerve endings was in the right elbow, which reacted positively to the stimulation. (TOS: "Spock's Brain")
In 3189, at the Hunhau salvage yard, a regulator injured the right elbow of prisoner Lai. He refused treatment for his elbow from Ryn. (DIS: "Scavengers")