A spectrum disorder is, in psychiatry, a disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits". The term "spectrum disorder" is heavily used in psychiatry and psychology, but has also seen adoption in other areas of medicine, for example hypermobility spectrum disorder and neuromyelitis optica spectrum disorder.