A piece of technology that allowed the recording and display of visual messages, not unlike holography. The D'ni people possessed that technology as well as the Amad on the Surface (possibly after some undocumented contact with the former).
Gehn possessed an Imager (possibly of Amad technology) where Keta had left him a love message. He carried it over to Riven and then to Age 233. On Riven, Gehn adapted the D'ni technology to mimic that of the Amad and created several Imagers. One could be seen in the school of the Jungle Island, while another of gigantic proportions was installed in the Temple of the Temple Island, obviously to command and intimidate the Rivenese.
The top level of the tusk in J'nanin had an Imager where Atrus had recorded mesages instructing his sons while teaching them The Art. The Imager was triggered only after his sons completed some of their tests. Saavedro overwrote most of these messages while preparing his revenge against Atrus.
An Imager was also to be found on Channelwood, perhaps constructed by Sirrus (and/or Achenar). Obviously Achenar used it to command and intimidate the Tree-dwellers, with his face or mouth appearing above a torture tool. Achenar could record his messages with a device in his room. Sirrus used it to leave a message to him.