Kiwix

Kiwix enables you to have the whole of Wikipedia (and many other web sites) available wherever you go! On a boat, in the middle of nowhere, or when data charges are too high, Kiwix gives you access to the whole human knowledge for free. You don't need Internet, everything is stored on your mobile device!

Download Kiwix from the iTunes App Store then download the ZIM data files which contain the content. You can download these files directly using the Kiwix App on your iPhone, or iPad - but this might take a long time, may incur expensive charges for the download, and is liable to errors because some files are rather large.

A faster and more reliable method is to use a computer to download the small torrent file for the large non-indexed ZIM file you want (not the pre-indexed package for Windows) from http://www.kiwix.org/wiki/Content_in_all_languages, then use a bit-torrent client (such a qTorrent) to download the actual ZIM data file to your computer. You can then transfer the ZIM file to your iOS device using iTunes File Sharing.