A joint, articulation (or articulate surface) is the location at which bones connect. They are constructed to allow movement (except for skull bones) and provide mechanical support, and are classified structurally and functionally.[page needed]
Joints are mainly classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.
Terms ending in the suffix -sis are singular and refer to just one joint, while -ses is the suffix for pluralization.
An articulate facet is generally seen as a small joint, especially used when speaking of the joints of the ribs.
Structural classification (binding tissue)
Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other. There are three structural classifications of joints:
fibrous joint - joined by dense regular connective tissue that is rich in collagen fibers 
synovial joint - not directly joined - the bones have a synovial cavity and are united by the dense irregular connective tissue that forms the articular capsule that is normally associated with accessory ligaments.
Functional classification (movement)
Joints can also be classified functionally according to the type and degree of movement they allow:
Joints can also be classified according to the number of axes of movement they allow, into monoaxial, biaxial and multiaxial. Another classification is according to the degrees of freedom allowed, and distinguished between joints with one, two or three degrees of freedom. A further classification is according to the number and shapes of the articular surfaces: flat, concave and convex surfaces. Types of articular surfaces include trochlear surfaces.
Joints can also be classified based on their anatomy or on their biomechanical properties. According to the anatomic classification, joints are subdivided into simple and compound, depending on the number of bones involved, and into complex and combination joints:
A joint disorder is termed an arthropathy, and when involving inflammation of one or more joints the disorder is called an arthritis. Most joint disorders involve arthritis, but joint damage by external physical trauma is typically not termed arthritis.
Arthropathies are called polyarticular when involving many joints and monoarticular when involving only one single joint.
Arthritis is the leading cause of disability in people over the age of 55. There are many different forms of arthritis, each of which has a different cause. The most common form of arthritis, osteoarthritis (also known as degenerative joint disease) occurs following trauma to the joint, following an infection of the joint or simply as a result of aging. Furthermore, there is emerging evidence that abnormal anatomy may contribute to early development of osteoarthritis. Other forms of arthritis are rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases in which the body is attacking itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint that results in subsequent inflammation. Additionally, there is a less common form of gout that is caused by the formation of rhomboidal shaped crystals of calcium pyrophosphate. This form of gout is known as pseudogout.