In medicine and psychology, the term syndrome refers to the association of several clinically recognizable features, signs (observed by a physician), symptoms (reported by the patient), phenomena or characteristics that often occur together, so that the presence of one feature alerts the physician to the presence of the others. In recent decades the term has been used outside of medicine to refer to a combination of phenomena seen in association.

The term syndrome derives from the Greek and means literally "run together", as the features do. It is most often used to refer to the set of detectable characteristics when the reason that they occur together (the pathophysiology of the syndrome) has not yet been discovered. A familiar syndrome name often continues to be used even after an underlying cause has been found, or when there are a number of different primary causes that all give rise to the same combination of symptoms and signs. Many syndromes are named after the physicians credited with first reporting the association; these are "eponymous" syndromes (see also the list of eponymous diseases, many of which are referred to as "syndromes"). Otherwise, disease features or presumed causes, as well as references to geography, history or poetry, can lend their names to syndromes.

A culture-bound syndrome is a set of symptoms where there is no evidence of an underlying biological cause, and which is only recognized as a "disease" in a particular culture.

Syndromes and associated conditions

The description of a syndrome usually includes a number of essential characteristics, which when concurrent lead to the diagnosis of the condition. Frequently these are classified as a combination of typical major symptoms and signs - essential to the diagnosis - together with minor findings, some or all of which may be absent. A formal description may specify the minimum number of major and minor findings respectively, that are required for the diagnosis.

In contrast to the major and minor findings which are typical of the syndrome, there may be an association with other conditions, meaning that in persons with the specified syndrome these associated conditions occur more frequently than would be expected by chance. While the syndrome and the associated conditions may be statistically related, they do not have a clear cause and effect relationship - i.e. there is likely to be a separate underlying problem or risk factor that explains the association. An example would be Down syndrome which has the associated condition of diabetes mellitus. A knowledge of associated conditions would dictate that they are specifically looked for in the management of the syndrome.

Case studies

One recent case study is Acquired Immune Deficiency Syndrome (AIDS), so named because most syndromal immune deficiencies are either genetically precoded, or secondary to either metabolic disorders or hematological disease. AIDS was originally termed "Gay Related Immune Disease" (or GRID), a name which was revised as the disease turned out to also affect heterosexuals. Several years passed after the recognition of AIDS before HIV (human immunodeficiency virus) was first described, finally explaining the hitherto mysterious "syndrome".