Transmat is simply the Terran term for matter transmission devices.
Transmats are not unique to the Terran Confederation, but they are a very rare technology, such that it often seems they are — they’re certainly the only race to currently use them so extensively. Most races that ever develop this technology tend to either never adopt it or else quickly abandoned it. The reason being, simply, that they’re exceedingly dangerous.
Various means and methods for matter transmission do exist throughout the galaxies, though all methods come down to a technique by which the relationship between energy and matter is exploited to allow some form of carrier wave to carry the matter across an intervening distance.
The danger of transmats comes in the fact that there is absolutely no simple, no-harm-done accident. When a matter transmitter goes wrong, it goes horribly wrong.
It has been suggested that, perhaps, minor mistakes do occur and in fact many who distrust transmats insist they happen with every transmission. Hence, for example, many top shelf vintners and chocolatiers refuse to allow their products to be moved by transmat; determined that it damages the flavours — most connoisseurs agree.
The other barrier to widespread adoption is that most sentient beings are in some fashion spiritual and/or religious. Of the worlds that will trust a matter transmitter, fewer still will use it to transmit living beings; the Terran Confederation and the Golfadre, both of the Milky Way galaxy, are the only races known to routinely transport themselves and others in this manner. The barrier is the very serious question: can a matter transmitter also transfer the soul? Even among the tri-galaxies’ less religious and even atheistic populous there is the question of: how do I know it’s me that comes out the other end?
In the TC these people are considered backward luddite alarmists in both cases. Among the Golfadre it is explained that the soul and the mind, already being energy, simply travel along with your mass and this seems to satisfy them.
It should be known that all forms of the technology are exceedingly sensitive to signal degradation so have common limitations. All are subject to unobstructed line of sight, various experiments with reflectors and repeaters have been attempted but are not successful at sufficient rates to have been adopted even by the most careless of societies. There are distance limitations as well, if transmitted at or near light speed using something like an electromagnetic wave the general limit is between one and thirty light seconds distance; this makes the technology viable for surface to moon or surface to space station transmissions plausible, given sufficient measures to reduce atmospheric disturbances to the signal (the TC precedes all atmospheric transmissions with a laser burst to clear a path, for example). Greater distances may be achieved, but at a marked loss of reliability if the matter is converted into faster than light energies (e.g. tachyons), also the gains tend to net few because while upper limits have been as great as a light hour there becomes a short-range limit — the signal never reconstructs properly under ten light minutes (the reasons for this are not yet properly understood).
A curious note in support of those who question the ability for the devices to deliver souls, spirits, intellects: non-organic goods transport with a full 30% greater reliability and non-living organic materials (e.g. leathergoods, foods, etc.) transport their own 30% more readily than living creatures. As an example, the TC has established transmat routes from Luna to the Jupiter, but will only transport goods and materials via this system. All living cargos must still travel by the stations on Luna, in Trans-Luna-Martian orbit, on Mars, to the Trans-Martian-Jovian orbital stations, and then to one of the Jovian satellites or orbital stations due to the safety concerns.
While various popular science fiction depictions of transmats will have them able to collect and deposit a subject anywhere all incarnations ever discovered by archaeologists and in present use require the subject to be sent by a transmitting device and reconstructed by a receiving device — in virtually all cases the technology was designed so that the same device could serve both functions. In the Terran Confederation, these devices are known as transmat capsules.