What primarily characterizes a fibrous joint?

A fibrous joint is primarily characterized as immovable because it is formed by dense connective tissue that tightly binds the bones together, allowing for little to no movement. This type of joint provides stability and strength, making it suitable for structures that need to resist considerable force. An example of a fibrous joint is the sutures in the skull, where the bones are interlocked and held together by a thin layer of connective tissue, preventing any movement between the bone plates.

The other options involve various degrees of mobility, but fibrous joints do not accommodate movement since their primary function is to maintain stability and rigidity in the skeletal structure.