Spectrum disorder in psychiatry is a term used to describe a mental disorder when there is thought to be "not a unitary disorder but rather a syndrome composed of subgroups" that can range from relatively "severe" to relatively "mild and nonclinical deficits". The term spectrum was originally used in physics to indicate an apparent qualitative distinction arising from a quantitative continuum (i.e. a series of colors formed when a beam of white light is dispersed by a prism so that their parts are arranged in the order of their wavelengths). In psychiatry, the concept of spectrum was first used with a slightly different connotation to identify a group of disorders that is qualitatively distinct in appearance (e.g. depression and alcoholism, or different types of mood disorder) but believed to be related from an underlying pathogenic point of view.

For different investigators, the hypothetical common pathogenic link has been of a different nature. Spectrum, dimension and continuum are related concepts. Some use them interchangeably while some argue the fine points of difference.

The term continuum suggests a simple straight line from severe to mild, while the term spectrum is used to indicate the fact that although there is a common denominator, a given type of individual may present with a given pattern of symptoms, reminiscent of the spectrum of distinct colors after refraction of light by a prism. In general, a spectrum approach is contrasted with a categorical approach, which is most associated with the DSM and ICD schemes. A spectrum approach may start with the nuclear, classic DSM diagnostic criteria for a disorder and then include an additional broad range of issues including temperaments or traits, lifestyle, behavioural patterns, and personality characteristics.

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