Robert J. Campbell
NOSOLOGY (from nosos, "disease") is the study of diseases from the point of view of their grouping, ordering, and relationship to one another; it includes the classification of diseases as well as the formulation of principles for differentiating one disease from another.
Diagnosis (from dia, "through, dividing into parts," and gnosis, "knowledge, recognition") is the process of distinguishing or recognizing the whole from its manifestations, of detecting the presence of disease from its symptoms. The process of diagnosis affirms that a disease is present; it defines the nature or character of that disease at the greatest level of specificity possible; and it provides a summary statement of what was discovered. Diagnosis is therefore both a process and a statement of the conclusion to which that process leads.
Nomenclature (from nomen, "name," and calare, "to call") is the agreed-upon label or wording that is used to communicate the results of the diagnostic process. Nomenclature is the shorthand name for the disease that has been identified, but in addition it implies that there is some reason for preferring one name to another.
Classification is the grouping of diseases into classes or orders, a logical scheme for organizing and categorizing so that different types of diseases can be distinguished and assigned their proper places.
All four terms—diagnosis, nomenclature, classification, and nosology—refer to various aspects of the conceptualization of disease. Because they are overlapping and interdependent, rather than mutually exclusive, it is not surprising that usage has tended to blur the distinctions between them. In itself, that is of little matter; what is unfortunate is that the vagueness and uncertainty that surround their use have spread as well over the assumptions on which they are based. Often lost sight of is that each of them reflects current speculation and hypotheses about the conditions to which they are applied and not only "hard" knowledge or scientific "fact."