Controlled vocabulary is a rather broad term, but here we mean by it a closed list of named subjects, which can be used for classification. In library science this is sometimes known as an indexing language. The constituents of a controlled vocabulary are usually known as terms, where a term is a particular name for a particular concept. (This is pretty much the same as the common-sense notion of a keyword.)
It is common to distinguish between term and concept by saying that the former is the name of a concept, and that the same concept may have multiple names, and also that the same term may name multiple subjects. A controlled vocabulary consists of terms, and not directly of concepts, and in general each term will be disambiguated to refer to a single subject (that is, there will be no duplicate terms). Note that “subject” as we have used the term so far is effectively equivalent to “concept”.
The term vocabulary also means slightly different things in the term “controlled vocabulary” from what it does in “metadata vocabulary”. The former is, as we noted, a set of indexing terms, or subjects used for classification, while the second is a set of properties of objects.
The purpose of controlling vocabulary is to avoid authors defining meaningless terms, terms which are too broad, or terms which are too narrow, and to prevent different authors from misspelling and choosing slightly different forms of the same term. Thus we can prevent authors from using “topic navigation maps” and “topic map” by forcing them to choose “topic maps”. We can also make it impossible to use “fnctional composition” as an indexing term instead of the correct “functional composition”.
The approach taken for IDEAlliance conference proceedings is what is known as an uncontrolled vocabulary, and this was abandoned recently in favour of a controlled vocabulary for precisely the reasons cited above. The simplest form of controlled vocabulary is simply a list of terms and nothing more. This is the approach currently used for the IDEAlliance conference proceedings, but more advanced schemes exist, as we will see below.