Final answer:
A synarthrosis is an immovable joint that provides a strong connection between bones for the protection of internal organs, such as the brain.
Step-by-step explanation:
In terms of function, the immovable joint is classified as a synarthrosis. These joints offer little to no movement, providing a secure union between adjacent bones, which is crucial in protecting internal organs. For instance, the sutures of the skull are synarthrotic joints that encase and safeguard the brain. Unlike synarthroses, amphiarthroses permit slight movement and diarthroses are freely movable joints, offering a wide range of motion.