A syndrome is a set of medical signs and symptoms that are correlated with each other. The word derives from the Greek σύνδρομον, meaning "concurrence". In some instances, a syndrome is so closely linked with a pathogenesis or cause that the words syndrome, disease, and disorder end up being used interchangeably for them. This is especially true of inherited syndromes. For example, Down syndrome, Wolf–Hirschhorn syndrome, and Andersen syndrome are disorders with known pathogeneses, so each is more than just a set of signs and symptoms, despite the syndrome nomenclature. In other instances, a syndrome is not specific to only one disease. For example, toxic shock syndrome can be caused by various toxins; premotor syndrome can be caused by various brain lesions; and premenstrual syndrome is not a disease but simply a set of symptoms.
If an underlying genetic cause is suspected but not known, a condition may be referred to as a genetic association (often just "association" in context). By definition, an association indicates that the collection of signs and symptoms occurs in combination more frequently than would be likely by chance alone.
Syndromes are often named after the physician or group of physicians that discovered them or initially described the full clinical picture. Such eponymous syndrome names are examples of medical eponyms. Recently, there has been a shift towards naming conditions descriptively (by symptoms or underlying cause) rather than eponymously, but the eponymous syndrome names often persist in common usage.
In medicine and psychology, a broad definition of syndrome is used, which describes a collection of symptoms and findings without necessarily tying them to a single identifiable pathogenesis. The more specific definition employed in medical genetics describes a subset of all medical syndromes.
In the field of medical genetics, the term "syndrome" is traditionally only used when the underlying genetic cause is known. Thus, Trisomy 21 is commonly known as Down syndrome. Until 2005, CHARGE syndrome was most frequently referred to as "CHARGE association". When the major causative gene (CHD7) for the condition was discovered, the name was changed. The consensus underlying cause of VACTERL association has not been determined, and thus it is not commonly referred to as a "syndrome".
In biology, "syndrome" is used in a more general sense to describe characteristic sets of features in various contexts. Examples include behavioral syndromes, as well as pollination syndromes and seed dispersal syndromes.
There is no set common convention for the naming of newly identified syndromes. In the past, syndromes were often named after the physician or scientist who identified and described the condition in an initial publication, these are referred to as "eponymous syndromes". In some cases, diseases are named after the patient who initially presents with symptoms, or their home town (Stockholm syndrome). There have been isolated cases of patients being eager to have their syndromes named after them, while their physicians are hesitant. When a syndrome is named after a person, there is some difference of opinion as to whether it should take the possessive form or not (Down syndrome vs. Down's syndrome). North American usage has tended to favor the non-possessive form, while European references often use the possessive. Even in Europe, there has been a trend away from the possessive form, over the period between 1970 and 2008.
Ibn Sina (Avicenna, 980-1037), in The Canon of Medicine, pioneered the idea of a syndrome in the diagnosis of specific diseases. The concept of a medical syndrome was further developed in the 17th century by Thomas Sydenham.
Even in syndromes with no known etiology, the presence of the associated symptoms with a statistically improbable correlation, normally leads the researchers to hypothesize that there exists an unknown underlying cause for all the described symptoms.