How can I use an iPad with a Retina display as an external monitor?
Update: There are now several competitors, such as Luna Display, as well as native support in iPadOS 13 + macOS Catalina, called Sidecar.



The app Duet Display lets iOS devices connect to a Mac that is running at least OS 10.9. The Mac needs duet software which is a free download and connects via Lightning port to the Mac in question to send the video.

The Mac app is a free download and the iOS app is available for purchase on the iOS App Store.