The PsycINFO database tags all content with controlled vocabulary, otherwise known as index terms. This standardizes the terms used across researchers and makes it easier for you to find relevant research. As an example, you can search for "Major Depression" which will automatically contain Dysphoria, Melancholia, and Depression.
When you open a search in PsycINFO, look for a link in the top, blue header that reads " APA Thesaurus of Psychological Index Terms".