Meaning and Need 

The term  'Vocabulary control' refers to a limited set of teal that must be used to index documents, and to search for these documents, in a particular system. It may be defined as a list of terms showing their relationships and used to represent the specific subject of a document. A certain degree of structure is introduced in a controlled vocabulary so that terms whose meanings are related are brought together or linked in some way. An uncontrolled vocabulary, is an unlimited set of terms drawn from natural language and used for describing the contents of documents. A natural language is  'natural' in the sense that it  grows freely, free from  any control whatsoever. Therefore, it is hardly possible to keep a natural language clear of  synonyms and homonyms. If  we use natural language for subject indexing, subject matter may be described by any one of the Words or phrases, without limitation,  such as those occurring in documents themselves. However, certain  problems  in  scotching do arise when no  control is imposed on the vocabulary. This is because of the fact that a natural language contains a large number of synonyms, quasi-synonyms, homonyms, acronyms, ambiguous terms, etc. Hence, if vocabulary control is not exercised, different indexers or the same indexer might use different terms for the  same concept on different occasions for indexing the documents dealing with the same subject and also use a different set of terms for representing the same subject at the time of searching. This, in turn, it would result in a 'mis-match' and thus affect information retrieval. Thus, the need exists for vocabulary control. In short, we can say that the  need for vocabulary control arises overcome -following problems: 

a)  occurrence of imprecisely defined words. 

b)  rapidly changing terminology. 

c)  numerous synonyms for a term. and 

d)  problem of homographs.