An inflammation of the fluid sac that lies between a tendon and skin or bone is known as what?

The condition described is known as bursitis, which refers specifically to the inflammation of a bursa—a small fluid-filled sac that acts as a cushion between tendons, bones, and skin. Bursae reduce friction and allow smooth movement at the joints. When these sacs become inflamed, often due to repetitive motion, injury, or prolonged pressure, it results in pain and discomfort in the affected area.

Tendinitis refers to inflammation of a tendon, which is distinct from bursitis as it involves the connecting tissue that attaches muscle to bone rather than the bursa itself. Arthritis relates to inflammation of the joints, often involving the protective cartilage, while dermatitis pertains to skin inflammation caused by allergies or irritants. Thus, bursitis is the appropriate term for the inflammation of the fluid sac mentioned in the question.