Instant Transmission (瞬間移動, Shunkan Idō, Instant Translocation, Teletransportation or Instantaneous Transmission) is a technique for instantly teleporting, taught to Goku on Planet Yardrat after his deadly battle with Frieza.
Instant Transmission allows Goku and anyone that is in contact with him (although planets do not count) to instantly travel great distances (from meters to light-years, or even travel between the living world and Other World) just by concentrating on a particular individual's energy signature, and 'transmitting' or feeling that being's location. This action is usually accompanied by Goku placing his index and middle fingers on his forehead to help him concentrate, though if extra time is taken the technique can be performed without this aid.
Because Instant Transmission requires a ki signature to lock onto (whereas Instantaneous Movement does not), the successful performance of the technique is dependent on one's ability to detect them. For example, at the end of DBZ, Goku proclaimed to Dende that he could not travel to New Namek because their ki signature was too far away.
The technique has many practical uses throughout the series. Goku can even shift through realms, having no problem teleporting from Earth to the Sacred World of the Kais in Other World, traveling there to find New Namek. In the Funimation dub of the anime, Goku describes the technique as allowing him to move at the speed of light (specifically 186,000 miles per second) however it occurs instantly as New Namek for example is light years away.
It is revealed in the eighth Dragon Ball Z movie, Dragon Ball Z: Broly - The Legendary Super Saiyan that this move briefly moves the user into a pocket dimension which then allows them to move to any location instantly. However, the user cannot travel to an area they have never been to unless they can detect someone familiar to them to use as a homing beacon.
When Goku became a child in Dragon Ball GT, however, the ability to use this technique was greatly hindered to the point where he just randomly travels to places a short distance away; however, as a Super Saiyan 4, he is able to use the ability to its full potential.
The beings on the planet Yardrat taught Instant Transmission to Goku during his time on the planet. He intended to use the technique to get to Earth before Frieza could harm anyone, but found it unnecessary because Future Trunks had already come to finish Frieza off. Later he discovered he can use two of his popular attacks with this technique, such as Instant Kamehameha and Instant Spirit Bomb. Throughout the rest of the series, Goku uses the technique often to travel around the Universe. In the Dragon Ball Universe, more of the heroes use the technique than the villains.
Goku can take passengers along with him, as long as they are touching him. However, in the episode "Piccolo's Decision", it is revealed that this ability has limits, as Goku said he could not move the entire world's population in one trip.
King Kai mentions he could have taught Goku the technique in the Cell Games Saga when Goku visited him on his planet to find the location of the New Namek (This is stated only in the anime, while in the manga, King Kai was amazed by the existence of such an ability).
Cooler's metal clone servants in the film Dragon Ball Z: The Return of Cooler also have the knowledge of Instant Transmission. His method of obtaining the technique is unknown, however it was stated that Cooler knew the technique before he came to Earth. Cooler used it to escape from the sun after Goku launched him there. Cooler is first seen using the technique during his second fight with Goku on New Namek. He revealed it after Goku used it to dodge a large energy bomb. Cooler states "that's quite a clever trick", and Goku responds "Yeah, it's called Instant Transmission", only to have Cooler blandly reply "I know...it's of one my favorite techniques". (In the original version, Cooler stated that he was surprised that Goku knew the move as well.) After this, there were times when both used it to fight; when used by both at the same moment, it resulted in the two battling in the dimension that makes up Instant Transmission. When this occurred, once, Cooler won the first skirmish in the Instant Transmission dimension by grabbing Goku by the leg and slamming him through a mountain; later, Goku entered it to save Vegeta from a crushing blow from Cooler, who had entered the dimension to surprise Vegeta. He also uses this technique occasionally in the story mode of Shin Budokai.
Cell learns this technique through absorbing Goku's cells after his self-destruction on King Kai's planet (near the end of the Cell Games Saga of Dragon Ball Z).
In the Cell Games Saga, King Kai states in filler that he knew the Instant Transmission technique and that he could have taught it to Goku if he had asked. However this conflicts with a previous statement he makes in the Vegeta Saga where King Kai specifically says that he couldn't "just zap" Goku back to Earth. Also, in the Frieza Saga King Kai stated that there was nothing he could do to help Goku escape the exploding Planet Namek, whereas if he knew Instant Transmission he could have traveled to Planet Namek and rescued Goku. Although, this could have been because both cases would have involved going to the living realm, something which only the East Supreme Kai is known to have done.