The Structure of Domain Terminologies in iQvoc
Domain terminologies are implemented based on the Simple Knowledge Organisation System (SKOS), a W3C recommendation.
SKOS distinguishes concepts from lexical labels. A concept is "an idea or notion; a unit of thought", represented by a globally unique identifier (an HTTP URI). A label is a term which is used to refer to a concept, technically "a string of UNICODE characters, such as 'romantic love' or 'れんあい', in a given natural language, such as English or Japanese". In most cases there are multiple labels denoting the same concept in the same language. One of them is the preferred label, while the others are alternative labels. If you have heard about "symmetrical vs. non-symmetrical multilingualism" (IFLA Guidelines for Multilingual Thesauri), the basic use case described by SKOS is symmetrical (each concept has a preferred label in each supported language), but the non-symmetrical approach (there is one concept per language with only one preferred label in this language) can be expressed as well.
There may also be a formal notation identifying a concept in some classification system such as the Dewey Decimal Classification. While a notation is a string of UNICODE characters just like a label, obviously a notation has no specific language. It has been taken from a specific classification system instead. There may be multiple notations from different systems for a single concept, just like labels from different languages.
Usually concepts will have a lexical definition, but this is not mandatory. There may also be several annotations, such as a general note, a change note, editorial note, history note, scope note, or an example. Technically, a lexical definition is just one kind of a note.
A more structural definition of a concept is given by its semantic relations to other concepts. Basically, we have only two types of relations in SKOS: hierarchical (broader, narrower) and non-hierarchical (related) relations. Both types are strictly disjoint: you will have to decide whether a relation is hierarchical or not, there must not be both relation types between the same two concepts. Some thesauri distinguish different types of hierarchy (as defined by Z39-19-2005): generic (a cat is a mammal), instance (Mimi is a cat), and whole-part (a cat has claws). SKOS does not follow this distinction, "broader" is defined as "more general in meaning", narrower as "more specific". Likewise there is no built-in assumption about transitivity of hierarchical relations. Both broader and narrower are not defined to be transitive, but the technical vocabulary provides means "to draw inferences about the transitive closure of the hierarchical relation".
The non-hierarchical relation is not further specified. This is due to the design goal of a simple knowledge organisation system. Any not-so-simple application may specialize different types of non-hierarchical relations, such as is-caused-by, in-contradiction-to, has-impact-on, or anything else. Doing so would exceed the limitations of a simple domain terminology and proceed to a more complex ontology. This may be a valid approach, but it is out of scope here.
Any set of semantically related concepts as described before can make a concept scheme. Examples are "thesauri, classification schemes, subject heading lists, taxonomies, 'folksonomies', and other types of controlled vocabulary", such as different domain terminologies.
Concepts from different concept schemes may have semantic relations as well, but these are called mapping relations. There are hierarchical (narrowerMatch, broaderMatch) and related (relatedMatch) mappings just like inside a single concept scheme, but there may as well be some equivalence (exactMatch) or similarity (closeMatch).
Finally SKOS defines a second pattern of describing sets of concepts: the collection. There may be any kind of use case "where a group of concepts shares something in common, and it is convenient to group them under a common label", but you do not want them to make a concept scheme. In fact a collection may even comprise concepts from different concept schemes, and collections may be nested, mixing collections and concepts on each level just like a file system.