MeSH, CINAHL Subject Headings, and other specialized thesauri combine controlled vocabulary with field searching to create highly relevant and comprehensive searches. In short, you are searching for a precise term in a precise place.
To illustrate, see this search using the keywords "cancer patient" and compared to this search using the Subject Heading (MH "Cancer Patients"). This number of results using the Subject Heading is roughly half. The results are also more specific, pertaining to patients with chronic cancer, which is defined in the Scope Notes.
Controlled Vocabulary = Terms that are clearly defined for a given database, catalog or similar resource. Classification systems, such as the DSM are very similar in that a clear specific definition of the terms is given for shared understanding.
Field Searching = Searching a specific section (field) of the metadata or item record. Most databases allow you to search by author, title, or publication. In CINAHL, there are search fields for Abstract, so you can search for terms that appear in the abstract. The Instrumentation search is for any surveys or instruments used in the study. For a complete list of search fields, go to the Advanced Search and open the Select a Field.
For more details see: CINAHL Help Sheet (EBSCO)