Final answer:
An immovable joint is known as a synarthrosis, which strongly binds the articulating bones together and plays a key role in protecting internal organs like the brain and heart through joints such as the skull sutures and manubriosternal joint.
Step-by-step explanation:
An immovable joint is classified as a synarthrosis. These joints are characterized by their inability to move, which contributes to a strong union between the articulating bones. This immobility is crucial in areas where the bones provide protection to internal organs. Notable examples of synarthroses include sutures, which are the fibrous joints found between the bones of the skull that protect the brain, and the manubriosternal joint, a cartilaginous joint connecting the manubrium and the body of the sternum, safeguarding the heart.