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In the SNOMED CT ontology, each concept is logically defined through its relationships to other concepts. For the definition of a concept, attribute-value pairs are used in addition to the hierarchical “is a” relationships. An attribute represents a characteristic of the meaning of a concept to which a value is assigned.

The attributes that can be applied depend on the concept model. The concept model defines which attribute value-pairs can be applied to which (sub)hierarchy of concepts and how those attribute-value pairs need to be grouped together to ensure correct unambiguous understanding of the concept definition (see the Editorial Guide for a detailed description of concept models). For example, a procedure may have a method, and a disorder may have an etiology, but a procedure cannot have an etiology, and disorder cannot have a method. Defining characteristics represent the values of a range of relevant attributes. Depending on the nature of the concept, there are more than 100 approved attributes to be used in the concept model. The permitted range of values for an attribute depends on the rules specified in the concept model.

All SNOMED CT concepts are linked at least by one is-a relationship to the immediate superordinate(s) concept(s) and by zero to many attributive relationships. Both types of relationships together form the (formal) definition of a concept. Relationships are not limited to concepts and can also refer to numerical values in products.

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