Target Display Mode explained.
And why it died.

What it was

You connected two Macs with a cable and pressed Command-F2 on the iMac's keyboard. The iMac stopped showing its own desktop and showed the other Mac's instead, like any external monitor. Press Command-F2 again and the iMac was itself again. Its speakers could play the other Mac's sound, chosen in that Mac's Sound settings.

For a while it was the official answer to "can my iMac be a monitor?"

The rules, in Apple's words

Apple's support page, Use iMac as a display with target display mode, lists three requirements. For the iMac used as a display:

Any iMac introduced in 2011, 2012, 2013, or mid 2014 using macOS High Sierra 10.13.6 or earlier. When using these iMac models as a display, the cable connecting the two Mac computers must be a Thunderbolt or Thunderbolt 2 cable.

Any 24-inch or 27-inch iMac model introduced in 2009 or 2010 using macOS High Sierra 10.13.6 or earlier. When using these iMac models as a display, the cable connecting the two Mac computers must be a Mini DisplayPort cable.

And for the Mac that connects:

Any Mac introduced in 2019 or earlier using macOS Catalina 10.15.8 or earlier. Target display mode does not support Boot Camp or Windows.

The same page adds that the other Mac can't use the iMac's camera or ports.

What that means per iMac

Three things trip people up:

• The iMac's own macOS. A 2012 or 2013 iMac that was updated to Mojave or Catalina is outside the rule, even though the hardware is the same.
• The connecting Mac. A MacBook Air or Mac mini with Apple silicon is never on the list. Neither is any Intel Mac introduced after 2019, nor one updated past Catalina.
• Adapters and docks. Plugable, a dock maker, says its docks and adapters do not pass Target Display Mode. The two Macs need a direct cable.

Why it died

Apple has never said why. What is on record: the 27-inch Retina 5K iMac from Late 2014 did not support Target Display Mode, no later iMac did either, and Apple's page caps the connecting Mac at 2019 and Catalina. The feature was never removed with an announcement. It simply stopped being included, and today's Macs fall outside it.

How to use it, if your setup qualifies

1. Check both Macs against the table above.
2. Connect them with the Thunderbolt or Mini DisplayPort cable listed.
3. Make sure you are logged in on the Mac that connects.
4. Press Command-F2 on the iMac. If the function keys are set as standard keys, press Command-Fn-F2.

If nothing happens, Apple suggests restarting both Macs, logging out of the iMac and trying again from the login window, or using the keyboard that came with the iMac: some third-party and older Apple keyboards don't send the shortcut.

What works with a Mac from 2020 or later

With a Mac you can buy today, Target Display Mode is off the table, whatever the iMac. What remains:

• Software over the network or a cable. Encore works with every Intel iMac from Late 2009 to 2020 (and 2007 to Early 2009 with Encore Receiver Legacy), over Wi‑Fi, Gigabit Ethernet or Thunderbolt, with any Mac on macOS 14 or later. Free during the public beta. Paid apps exist too, each with its own iMac range.
• AirPlay to Mac, if the iMac is from 2019 or later and runs macOS 12 Monterey or later.
• Hardware, such as a capture card, or replacing the iMac's insides with a display driver board.

We compared all of them, with sources, in every way to use an old iMac as a monitor, and listed what replaces it in Target Display Mode alternatives for Apple silicon. Side by side with Apple's rules: Encore vs Target Display Mode. To see what your model can do, start with the year-by-year guide or the compatibility page, and the cable guide if you want to plug in.