A temporal communicator was a device which could be used to communicate with other time periods.
Temporal communications to inhabitants of an earlier time frame were considered dangerous to the integrity of time; they were prohibited by the Temporal Prime Directive in the Federation.
Communications from the future contributed to the Temporal Cold War, as future benefactors would send advantages and missions to be carried out by the Cabal.
In the 31st century, Federation temporal communicators were dependent on a component called a quantum discriminator. (ENT: "Shockwave, Part II")