A Holotransmitter was a type of technology that made use of Holograms.

Consisting of a module that mixed in elements of transporter technology with high tech communications systems. The holotransmitter was capable of creating realistic holograms which was projected from the device and allowed for communication with individuals over long distances which was accomplished through the use of holographic avatars. Activating the device led to the individual in question suddenly appearing allowing for easy communication with them.

Whilst considered by some to be impractical over long range transmissions, these devices were not capable of being detected easily as they made use of a piggy back technique that rode the existing carrier waves making it virtually undetectable. During its creation by Starfleet in the early part of the 24th century, the technology was considered to be decades away before it could become standard issue. Doctor Scott Heisenberg was the chief developer of this form of technology.

By 2360, Starfleet Intelligence had developed a prototype version of this device during the Gnawing incident where the holotech was demonstrated to a young Lieutenant Benjamin Sisko. (TOS novel: Catalyst of Sorrows)

Essentially, the device appeared to be a form of small mobile Holoemitter.
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