MediaWiki:Apihelp-query+prefixsearch-description/en

Perform a prefix search for page titles.

Despite the similarity in names, this module is not intended to be equivalent to Special:PrefixIndex; for that, see action=query&list=allpages with the apprefix parameter. The purpose of this module is similar to action=opensearch : to take user input and provide the best-matching titles. Depending on the search engine backend, this might include typo correction, redirect avoidance, or other heuristics.