The use of controlled vocabulary aims to create consistency in metadata, so that researchers publishing and researchers searching can use the same vocabulary to index and find papers, results, and data. 

Controlled Vocabulary and Taxonomies are standardization practices to create consistent, sometimes formulaic, ways to describe objects, phenomena, and/or processes within a field of study. 

Controlled vocabularies are a similar concept to data dictionaries in GNSS/GPS data collection, or domains in databases. 

Examples

• ISO 3611, ISO 4217, and ISO 639 are three examples of simple vocabularies, they define country codes, currency codes, and language codes respectively. These codes are used around the world to exchange information across systems. 
• The EU Publications Office manages vocabularies to standardize names and concepts across the 20+ languages the EU publishes documents in. Controlled vocabularies – Publications Office of the EU 
• Consortium of European Social Science Data Archives (CESSDA) has created a vocabulary service to standardize social science research data. Their vocabulary includes definitions for language fluency, roles of persons in research, data types, data collection instruments, research lifecycle events, and many more. Browse their vocabulary here.  
• The General Multilingual Environmental Thesaurus (GEMET) is a vocabulary set for describing geographic and environmental data, it includes definitions and official translated vocabulary, and is a project of the EU INSPIRE directive. INSPIRE Spatial Data Themes (europa.eu)