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, but in a minority of people, pain and other symptoms are present. This may be a sign of what is known as joint hypermobility syndrome (JMS) or, more recently, hypermobility spectrum disorder (HSD). Hypermobile joints are a feature of genetic connective tissue disorders such as hypermobility spectrum disorder (HSD) or Ehlers–Danlos syndromes (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.

In 2016 the diagnostic criteria for hEDS were re-written to be more restrictive, with the intent of narrowing the pool of hEDS patients in the hope of making it easier to identify a common genetic mutation, hEDS being the only EDS variant without a diagnostic DNA test. At the same time hypermobility spectrum disorder was renamed as hypermobility spectrum disorder and redefined as a hypermobility disorder that does not meet the diagnostic criteria for hEDS, other types of Ehlers-Danlos Syndrome, or other heritable Connective Tissue Disorder (such as Marfan's, Loeys-Dietz, or osteogenesis imperfecta).