Later in life, when growth is complete they fuse. The joint is maintained by the periodontal ligament which gives only a little to act as a shock absorber when we bite on a ball bearing. In this type there is a little movement, limited by a tight ligament. Since many joints are limited by ligaments this is probably a special definition we can do without. The symphysis pubis with ligaments and fibrocartilage is normally closed, but opens in childbirth due to hormonal influences.
Synovial joints have different parameters.
Joint surfaces almost in contact but discontinuous, as a great range of movement is often possible, and the surfaces slide over each other. The sliding surfaces are covered with a thin layer of cartilage. The joint cavity is sealed by a synovial membrane which secretes synovial fluid, a lubricant and nutrient. Around this, in turn, is a tough fibrous joint capsule which keeps the ends of the bones in proper orientation. This is often locally thickened to form joint ligaments.