A senceiver was an experimental device being tested by Starfleet Command in the early 2270s. The senceiver was implanted inside an individual's head, and acted as a communications medium, allowing text and visual messages to be transmitted into the person's brain. The use of the senceiver was somewhat controversial, with some worried that the technology could be put to use for mind control.
In 2273, Admiral James T. Kirk was able to receive the visual recording made of a battle between three Klingon K't'inga-class battle cruisers and the V'Ger entity. (TOS novelization: Star Trek: The Motion Picture)
By 2280s, only flag officers serving planetside were required to have senceivers, and used primarily as locaters rather than communication devices. Still, Dr. Leonard McCoy hesitated to use this device when trying to track down Admiral Kirk, considering it an invasion of privacy. (TOS novel: Strangers from the Sky)