Can you use iMac Retina 5K as external monitor for a MacBook Pro? I remember when thunderbolt came out there was a lot of discussion of how it had interesting magical bi-directional capabilities. 

Does that mean there is a way to use an iMac Retina 5K as an external, secondary monitor for a recent MacBook Pro?
According to Apple, no. The iMac Retina 5K does not support Target Display Mode.


  Note that the iMac (Retina 5K, 27-inch, Late 2014) does not support Target Display Mode. 


You can see it here.