Synovial fluid is the fluid found between the cartilage in joints between bones. It lubricates the movement between bones and prevents friction damage to the cartilage. The fluid layer is very thin, typically about 50 microns thick (one-half of one millimeter). In addition to this separation layer, it fills any imperfections in the cartilage. Unlike many of the body's fluids, which are very watery, synovial fluid is rather more viscous, similar in consistency to an egg white. It's secreted from a membrane next to the joint.
The basis of synovial fluid is plasma to which the membranes add several other proteins.
In addition to reducing friction, the fluid also acts as a shock absorber, provides nutrients to the surface, removes waste from the surface, and prevents foreign matter from getting into the space.
In most inflammatory diseases of the joints, like arthritis, the synovial fluid is more watery.