Hypermobility Syndrome as defined by The Hypermobility Association (UK):

Connective tissue proteins such as collagen give the body its intrinsic toughness. When they are differently formed, the results are mainly felt in the "moving parts" - the joints, muscles, tendons, ligaments - which are laxer and more fragile than is the case for most people. The result is joint laxity with hypermobility and with it comes vulnerability to the effects of injury.

The Hypermobility Syndrome is said to exist when symptoms are produced, a state of affairs that may affect only a minority of hypermobile people. It is probably more correct to refer to Hypermobility Syndromes (in the plural) as a family of related genetically-based conditions which differ not only in the particular protein affected, but also in the degree of difference of formation. Thus at one end of the spectrum are the diseases with the potentially serious complications such as Marfans Syndrome or Ehlers-Danlos Syndrome Vascular Type (formally EDS IV). At the other end are what is now called on good evidence Benign Joint Hypermobility Syndrome (BJHS) and Ehlers-Danlos Hypermobile Type (formerly EDS III), which may be one and the same. These may cause troublesome and persistent problems, but do not affect the vital organs and thus do not pose a serious threat to life.