What term refers to the abnormal stiffening and immobility of a joint due to the fusion of bones?

The term that refers to the abnormal stiffening and immobility of a joint due to the fusion of bones is "ankylosis." This condition results when the bones in a joint become fused or rigidly connected, which leads to the loss of movement in that joint. Ankylosis can occur as a result of various underlying conditions, including inflammatory diseases, injury, or age-related changes, and it commonly affects joints such as the spine or hips.

Osteoporosis, on the other hand, is characterized by a decrease in bone density, making the bones more fragile and prone to fractures rather than causing stiffening of the joints. Arthritis refers to inflammation of the joints, which can lead to symptoms such as swelling, pain, and reduced mobility, but does not necessarily imply fusion of bones. Rheumatism is a broad term that encompasses various conditions affecting the joints and connective tissues, but it does not specifically refer to the fusion of bones. Therefore, ankylosis is the most accurate term to describe the stiffening and immobility of a joint due to bone fusion.