Kiwix is an open source tool which allows you to download Wikipedia content to PCs, Macs, Linux or Android devices, and browse it offline.
Agree to download the list of library files when the program first launches, click "Get new files", and you'll see a list of available content. When we checked the list, the main file was "Wikipedia", a 9.68GB download containing all the site's English language articles (some 3.87 million, in total). But there are also smaller editions - the "45,000 best articles", or "6000 Wikipedia articles for school children" - along with extras like Wikivoyage ("The free worldwide travel guide") and more.
Download any file you like, load it in your library, and you can browse it just like regular Wikipedia. Run a search, or browse the index; click a link, and the relevant article appears, complete with images, ready for you to review.
One problem here is these offline editions are a little out of date. The major Wikipedia download is dated March 2012, for instance; that's probably fine if you're researching geography, history or similar topics, but it's much more of an issue if you're browsing pages for music, TV, film, or anything else which might see regular updates on a daily basis.
If you can live with that, though, Kiwix provides a simple way to access and search Wikipedia content offline.