Hypermobility, also known as double-jointedness, describes joints that stretch farther than normal. For example, some hypermobile people can bend their thumbs backwards to their wrists, bend their knee joints backwards, put their leg behind the head, or perform other contortionist "tricks". It can affect one or more joints throughout the body.

Hypermobile joints are common and occur in about 10 to 25% of the population. In a minority of people, pain and other symptoms are present. This may be a sign of hypermobility spectrum disorder (HSD). Hypermobile joints are a feature of genetic connective tissue disorders such as hypermobility spectrum disorder or Ehlers–Danlos syndrome (EDS). Until new diagnostic criteria were introduced, hypermobility syndrome was sometimes considered identical to hypermobile Ehlers–Danlos syndrome (hEDS), formerly called EDS Type 3. As no genetic test can distinguish the two conditions and because of the similarity of the diagnostic criteria and recommended treatments, many experts recommend they be recognized as the same condition until further research is undertaken.