Synovial fluid is a clear, viscuous fluid with a stringy, egg-like consistency (the Latin synovium means "with egg") found in synovial joint (freely-moveable joint) cavities, bursae, and tendon sheaths. It acts as a lubricant, reducing the friction between articular cartilage and other tissues in joints, and also as a shock absorber during movement. Synovial fluid is secreted by synovial membranes.

The viscous and elastic properties of synovial fluid are due to hyaluronan, a long chain glycosaminoglycan carbohydrate with a molecular mass of about 1 million.