Despite its massive size, it is possible to run your own offline version of Wikipedia (or just read certain articles offline in a convenient form). A popular option for the sole user is Kiwix, however, if you want to run a server for multi-user access or want a cross-platform option, Gozim is worth a look.
Written in Google's Go web language, Gozim is a free, open-source ZIM reader that can download, index and serve Wikipedia, as long as you're willing to download the required files. You can grab the whole thing, images included, if you have 40GB of bandwidth to spare. Alternatively, if you just want the text, 12GB is all you need.
What makes Gozim different to other offerings is that you can run it on just about anything, including a Raspberry Pi, tablet or smartphone. All you have to do is run the included indexer program (if you want article searching) and then fire up the web server. You should then be able to browse to the device — and the offline Wikipedia — by typing in the IP address and port (8080 by default).