Super Saiyan

Super Saiyan is an advanced transformation assumed by members and hybrids of the Saiyan race with sufficient amounts of S-Cells in the Dragon Ball franchise, though there have also been occurrences of other races or individuals taking on their own versions of the form. Earthlings (of Saiyan heritage) in Dragon Ball Online can also assume the transformation by wishing to have their dormant Saiyan powers unlocked.

Legend
The ability to become a Super Saiyan was once considered to be nothing but a legend, as it had not been performed for over one-thousand years. Vegeta stated that a Super Saiyan has achieved a level of power so overwhelming that it could only be maintained in a transformed state. Despite only being told through legend, Frieza, who had Saiyans in his army, became paranoid over the possibility of truth behind the legend due to the constantly growing power of the Saiyan race. After receiving informal permission from the God of DestructionBeerus, Frieza finally decided to annihilate the Saiyan race by destroying Planet Vegeta, thus apparently sparking the end of the Super Saiyan legend. However, twenty-five years later, the Super Saiyan legend was proven to be true when Gokuaccomplished the feat during his titanic battle against the evil tyrant, Frieza.

Transformation
It was originally believed that the Super Saiyan transformation comes in response to a need, not a desire. Any Saiyan can become a Super Saiyan, however they require a large amount of S-Cells in order to take on the form, Saiyans who have a gentle spirit gain more S-Cells, but a when a Saiyan increases their battle power they steadily gain more S-Cells. Once the Saiyan has enough S-Cells, a feeling such as strong anger or sadness can transform a Saiyan into a Super Saiyan, once a Saiyan has achieved the form they can transform into it any time by focusing their energy into their back to make it tingle. A Saiyan who has a very large battle power can turn Super Saiyan without needing a trigger, due to their tremendous amount of S-Cells. Saiyan offspring will inherit S-Cells from their parents, this is why Goten, Trunks and the hybrid clone Cell were able to become Super Saiyans so easily.

Goku achieves the form out of anger over the murder of his best-friend Krillin and in a what-if scenario Bardock achieves the form out of anger after Chilled attacked a Plant child called Berry, while Vegeta achieves the form through sheer frustration at his original inability to achieve it, especially when Goku already had. While training in the Hyperbolic Time Chamber, Goku informs Gohan that becoming a Super Saiyan is powered by rage and told him to imagine Cell and then Frieza (after Gohan stated he had never seen Cell) killing all his friends in order for him to transform into a Super Saiyan. As the Saiyans train during Dragon Ball Z, they discover uncharted levels beyond the first Super Saiyan level. The power increases of subsequent Super Saiyan forms are so high that by the Buu Saga, the original transformation has been literally reduced to that of a "child's plaything" (as stated by Vegeta; as both Goten and Trunks can transform and harness the power adeptly at such a young age).

In the main series, all living Saiyan-blooded males after the Frieza Saga manage to reach at least the first level of Super Saiyan. No female Super Saiyans were seen in the Dragon Ball manga or anime until Caulifla achieved the transformation in Dragon Ball Super.

However, the Dragon Ball GT Perfect Files implies that Pan has the potential to transform and might have become a type of Super Saiyan never seen before. Female Super Saiyans have since appeared in video games, including Dragon Ball Online, Dragon Ball Heroes and Dragon Ball Xenoverse.

Overview
In Dragon Ball Super, while explaining Goku's Super Saiyan form to Beerus as they were watching parts of Goku's battle with Frieza, Whis mentions that the Saiyan's knowledge of the form was lost to their race at some point in the past, resulting in it being viewed by most Saiyans as little more than a legend until it was rediscovered by Goku and the surviving Saiyans. The Namekian Grand Elder Guru also showed some awareness of the existence of Super Saiyans when he mistakenly thought that the Nameless Namekian had been killed by a Saiyan while speaking to Krillin about Kami and Piccolo's deaths, unaware of their split until after he probed Krillin's mind. Guru implied that only a Super Saiyan would have the power to kill a Super Namek like the Nameless Namekian. This indicates that Namekians may have encountered Super Saiyans in the distant past or had heard about them during the time the Namekian people were an interstellar race, as Guru possessed knowledge of them and the legend concerning the Original Super Saiyan God recorded in the Namekian Book of Legends.

In Dragon Ball GT, Old Kai was shown to be aware of both the Golden Great Ape and Super Saiyan 4 forms which are only accessible to Saiyans who have achieved the Super Saiyan form yet also capable of transforming into a Great Ape, indicating that Super Saiyans, Golden Great Apes, and Super Saiyan 4 had previously existed at some point in the past for Old Kai to possess knowledge of those transformations.

In the Dragon Ball Super manga it is shown that when a Super Saiyan's power surpasses the power of the Super Saiyan God form, it will naturally progress into a Super Saiyan God Super Saiyan.