Index terms are topic-relevant standardized search terms that are tagged (i.e., indexed) as topic-relevant by each bibliographic source. These terms are found at the beginning of search filters created by filterNHP if they are relevant to the taxonomic group you are interested in. PubMed uses MeSH terms (i.e., Medical Subject Headers) created by the National Library of Medicine (NLM) and are searched using the syntax '[mh]' or '[mh:noexp]'. Similarly, PsycINFO uses the American Psychological Association (APA) Thesaurus of Psychological Index Terms and are searched for via EBSCOhost within the syntax 'DE()'. Web of Science does not have an associated thesaurus. Each bibliographic source organizes index terms into a hierarchical structure by subject category so that more specific terms (i.e., narrow) are nested under more general terms (i.e., broad). Including index terms expands the scope of literature searches and may result in the retrieval of publications that may have gone undetected otherwise.