An iMac feature that lets the screen function as an external monitor for another computer. Thunderbolt-based iMacs starting in 2011 can only be a Target Display Mode (TDM) screen for computers connected via Thunderbolt. Previous Mini DisplayPort-based iMacs could serve as an external screen if connected via Mini DisplayPort, as well as Thunderbolt, HDMI and DVI with the appropriate adapter cables. See
Target Disk Mode.