Technically and in the wiki context, FullHistory means a complete archive of page revisions (KeptPages). Such a FullHistory may be used as a safety net, for scientific purposes or to reconstruct authorship. Implementations can be done efficiently using systems like RCS, especially if you use some filter for smaller changes of the same author. Engines that implement FullHistory: MoinMoin, ProWiki, UseModWiki, MediaWiki (with $KeepDays = 9999999).
A FullHistory allows for interesting secondary benefits, like a WayBackMode, the HistoryFlowVisualization, and unobtrusive ethnographic analysis.