To do a good search, it is important to use the right terms. Most databases use subject words to describe the contents of the documents. The same document can appear in several databases with different subject words, depending on the database content.
Etymology: Latin, < Greek θησαυρός a store, treasure, storehouse, treasury.
Oxford English Dictionary
The subject words are referred to differently in the databases. Some databases use the term descriptors, subject terms or IEEE terms. In Medline/PubMed, the index terms are called MeSH.
A thesaurus contains controlled index terms, i.e. words deliberately chosen to describe a document in a certain database. Using an index term from a thesaurus in your search will generally generate highly relevant search results.
In the thesaurus, the subject words are arranged hierarchically from broader to narrower concepts. There are also references to find the right words. Tips on related subject words may also be available.
Many databases have an extensive thesaurus. If so, the thesaurus is a good starting point for a search. The thesaurus is often available through the database under headings as Thesaurus, Subjects, MeSH or CINAHL Headings.
Free text searches
Other databases have a simpler vocabulary or lack a dictionary. Then you have to find your own keywords and perform free text searches.