||The English used in this article may not be easy for everybody to understand. (December 2011)|
Continuum theories or models explain variation as involving a gradual quantitative transition without abrupt changes or discontinuities. It can be contrasted with 'categorical' models which propose qualitatively different states.
In physics, for example, the space-time continuum model explains space and time as part of the same continuum rather than as separate entities. A spectrum in physics (e.g. of light) is often termed either a 'continuous spectrum' (energy at all wavelengths) or 'discrete spectrum' (energy at only certain wavelengths).
In psychology, theories of mental phenomena can propose discrete differences between individuals (e.g. everyone has certain personality traits and not others) or a continuum (e.g. everyone lies somewhere on a particular personality dimension). This can also apply to fields such as law or sociology or ethics in explaining or judging variation in human behavior.
In psychiatry, categorical models seek to distinguish and define particular mental disorders or illnesses, but continuum or dimensional models propose that some people are more extreme than others on particular dimensions.