Kiwix is an offline content reader which can be used for Wikipedia as well as other contents (Ubuntu documentation, WikiLeaks, WikiSource, WikiVoyage, etc).
Once you have downloaded the content file (potentially very large), you don't need any connection to use it.
Kiwix is a lightweight software reading ZIM files which you download and store on your device or external storage (SD card).
Please check the instructions inside the App or on the website to learn more about available contents.
Note: Kiwix is also available on regular computers (Windows, Mac, Linux).