Sideloading

Sideloading is the process of installing software from unofficial sources, either from external storage media or downloaded from the open Internet. Apple discourages downloading software from unknown sources as this could put a system at risk of a malware attack.

Sideloading on iOS
Downloading software from a source other than the Apple App Store to an iOS (or iPadOS) device requires jailbreaking. Apple CEO Tim Cook states that such security measures are in place to protect user data and privacy. Cook has openly recommended Android for users who want to sideload apps.

Sideloading on macOS
By default, macOS now limits software installation to users with administrator privileges. Starting in OS X 10.11 (El Capitan), the Gatekeeper feature of Security & Privacy preferences began hiding the option to allow apps downloaded from Anywhere. This can be allowed on a one-by-one basis through the preference pane, or re-enabled with the following Terminal command: (both steps require admin access)

sudo spctl --master-disable

Apple senior VP Craig Federighi stated that such steps are necessary to protect macOS from the "malware problem". He stated that while tech savvy users may not be harmed by sideloading, this may not be true of casual users and children.