When stretching, a lubricant called synovial fluid is released from the joint cavity. (rejuvmedicalky.com)
Synovial fluid: it is secreted by the synovial membrane and fills the joint cavity; it provides lubrication to the joint, reduces bone erosion and aids in joint mobility. (altichealthprevention.com)
Joints are made up of a joint cavity filled with synovial fluid, a synovial membrane, and particular cartilage. (puppy-basics.com)