A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.

Some joints, such as the knee, elbow, and shoulder, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements.

[3] Other joints such as sutures between the bones of the skull permit very little movement (only during birth) in order to protect the brain and the sense organs.

They are essential for mobility and flexibility, connecting bones and facilitating a wide range of motions, from simple bending and stretching to complex actions like running and jumping.

Beyond enabling movement, joints provide structural support and stability to the skeleton, helping to maintain posture, balance, and the ability to bear weight during daily activities.

Rheumatoid arthritis, an autoimmune disorder, causes chronic inflammation in the joints, often resulting in swelling, pain, and potential deformity.

Another prevalent condition, gout, arises from the accumulation of uric acid crystals in the joints, triggering severe pain and inflammation.

Effective treatment and management of joint-related conditions often require a multifaceted approach, including physical therapy, medications, lifestyle changes, and, in severe cases, surgical interventions.

[3] Joints can also be classified, according to the number of axes of movement they allow, into nonaxial (gliding, as between the proximal ends of the ulna and radius), monoaxial (uniaxial), biaxial and multiaxial.

[21] Humans have also developed lighter, more fragile joint bones over time due to the decrease in physical activity compared to thousands of years ago.