What are controlled vocabularies?
Language is fluid. A singular concept can be expressed in abbreviation, using variant spellings, and using variant terms. Controlled vocabularies aid searching by including all the alternative ways a concept is expressed.
For example, bovine spongiform encephalopathy can additionally be expressed as BSE, Bovine Spongiform Encephalitis and Mad Cow Disease.
Controlled vocabularies are not used in internet search engines or even all library databases, but when available, can be extremely handy. Why? They are more useful than the Boolean Operator OR for catching all synonyms and variant spellings for a search term. Plus, you can restrict/focus your search term. Meaning? Instead of finding every article that mentions diabetes, you can find only articles where diabetes is the major focus/topic.
Below are databases with controlled vocabularies: