WikiEngine is the thing that's making this thing work. It's the base driver class for JSPWiki's wiki engine, providing wiki services to the custom JSP (Java Server Pages) pages. It consists of a few hundred lines of Java code, and handles things like converting the Wiki markup into glorious HTML for you to view, creating and maintaining access to various internal managers, storing pages, etc. The actual HTML translation is done by a custom java.io.Reader, called TranslatorReader, and the page storage is done via pluggable WikiPageProvider classes.
Always get yourself an instance from JSP page by using the WikiEngine.getInstance() method. Never create a new WikiEngine() from scratch, unless you're writing tests.
There's basically only a single WikiEngine for each web application, and you should always get it using the WikiEngine.getInstance() method.