Final answer:
A synarthrosis is an immobile joint that provides a strong union between bones, playing a role in protecting internal organs such as the brain and heart.
Step-by-step explanation:
A synarthrosis is a type of joint that is characterized by its immobility or very limited mobility. These joints provide a strong union between articulating bones and are crucial in protecting internal organs. Examples include the sutures in the skull, which protect the brain, and the manubriosternal joint for heart protection.