Utilise dictionaries, ontologies, lexicons, and thesauri when searching for information in databases. Note that databases can have their own inbuilt lexicons for you to use. The search terms in these lexicons are often referred to as "Subject Terms", "Subject Headings", or "Descriptors".
MOT Dictionaries 
Finto Vocabularies and ontologies (Finnish, English, Swedish)
AFO Agriculture and Forestry Ontology
EnDic Environmental dictionary
LCHS Library of Congress subject headings