Subject indexes and subject terms can be useful in many ways when choosing search terms and performing searches in databases. A controlled subject index is also known as a thesaurus. A thesaurus is often a specialized glossary for a certain discipline. Glossaries can also be general in character, independent of databases and subject area.
Thesauruses describe the relationships between the subject terms (or descriptors) i.e. broader, narrower and related terms. In the case of synonyms, thesauruses often tell you also what descriptor is used for a particular concept. In databases contents of documents are described with subject terms selected from the thesaurus used in that database. When performing searches it is possible to focus a search to look for only these subject terms in the subject term field. Thesauruses and descriptors in them are database specific and must therefore be checked from database being used.
The metallurgical thesaurus in ProQuest presents, in addition to the descriptors, broader terms and narrower terms as well as related terms.
Image source: ProQuest <http://search.proquest.com> 25.7.2013
Thesauruses often tell you also what descriptor is used for a particular concept. As can be seen in the thesaurus from the ProQuest database,vision systems is used as the descriptor for the term machine vision in this database, whereas the Ebsco Academic Search uses the term computer vision as the descriptor.
Image sources: EBSCO <http://web.ebscohost.com> 25.7.2013, ProQuest <http://search.proquest.com> 25.7.2013
If only a few references are found, it is advisable to use a broader term from the thesaurus. On the other hand, if there are many references, it may be wise to use a narrower term in the search. Note that when we talk about a related term, this does not mean that it is a synonym.
Examples of thesauruses: VESA The VESA Finnish General Thesaurus (Finnish National Library), here is also a link to other thesauruses.