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In the following, we will give a summary description of concepts and terms, concept systems, and definitions. Please see the international terminology standard ISO 1087-1:2000 for a systemic description of these notions.

2.1 Concepts and terms

The semiotic triangle (Figure 1) introduced by Ogden and Richards (1923/1989) is of central importance in terminology work. This triangle is a model of how symbols are related to the objects they represent. It consists of 3 elements: concept (“Thought or Reference”), designation (“Symbol”) and object (“Referent”). A concept is the mental image of concrete or abstract objects in reality and consists of characteristics based on properties observed in an object or in its relationship to other objects. Concepts are represented by a designation (symbol) such as words (in general language), terms (in specialized language) or other perceivable notations (e.g. graphics, numbers, etc.). In SNOMED CT the concepts are represented by so-called Descriptions (see Section 3.4).


The dotted line between the referent (object) and the symbol (description) indicates that there is no direct connection between the two, as the object must be conceptualised before it can be rendered. There is also no one-to-one (i.e. one term refers to one concept) but a many-to-one relation between concept and term (different terms refer to one and the same concept). Examples are the English terms “typhoid fever” and “infection by Salmonella Typhi” for the concept |typhoid fever|. This is called synonymy, which is a frequent phenomenon in clinical terminology. Conversely, one term can refer to different concepts (i.e. one and the same term has different meanings), which is called polysemy. For example, the term “drug” refers, depending on the context, to the following concepts: (1) |substance used for diagnostic/treatment/ prevention purposes|; (2) |chemical substance used as narcotic or hallucinogen|; (3) |substance used in dyeing or chemical operations (obsolete)|.

2.2 Concept systems

The main goal of terminology work is to clarify and standardize concepts and terms to facilitate unambiguous communication in natural (human) language. Concepts are related to other concepts (conceptual relations) and form networks or concept systems, which are extremely useful in many contexts: they provide an overview of a subject area and make it possible to delimit and define concepts, to form new descriptions, to evaluate existing and competing descriptions, and to structure systematic representations of a given domain. Since concept systems are important tools for analysing and translating concepts, they play a crucial role in terminology work.


Figure 2 – Hierarchical and attribute relationships in SNOMED CT; example with the stated view of concept 18027006 |Transplantation of liver (procedure)| (International Edition 2021-07-31)

2.3 Definitions

In terminology work, the definition of a concept is extremely important. A definition describes the characteristics of a concept to delimit it from neighboring concepts. Definitions are therefore essential for the clear assignment of terms to the underlying concepts. There are different types of definitions (analytical, encyclopaedical). Definitions can be written in natural language or in a (semi-)formal language.