Databases often have an index (=alphabetical list) about the contents of different fields. The indices contain terms in the same form as they were used in the particular database, and they do not follow any hierarchy, as words in a thesauruses do. Indices can be used for finding suitable search terms which describe the topic, or they can be used for a spelling check.
In the Medline database, almost every searchable field has an index that can be browsed by selecting the desired fields. Image source: Ovid <http://ovidsp.uk.ovid.com/sp-3.8.1a/ovidweb.cgi> 18.7.2013
You can choose suitable terms from the index, and use them to run a search. In this example, articles written by Kari I. Kivirikko are searched for. An alphabetical list of authors is useful when wanting to check the spelling of names. Also spelling mistakes can creep into an index. Image source: Ovid <http://ovidsp.uk.ovid.com/sp-3.8.1a/ovidweb.cgi> 26.7.2013
