Most periodical indexes organize their records using a list of approved subject headings. This allows you to find all the articles about your topic under one term.
How do you find the approved subject headings? Some indexes provide a list of subject headings. This list is called a subject guide or a thesaurus. If you search for a word that is not in the index, it may suggest an approved subject heading you could use.
If you looked for articles using the words Online advertising, this database would direct you to use the subject heading Internet advertising.