Saiyan's possess a genetic trait that allows their power to increase substantially after recovering from near fatal injuries. This trait is not exclusive to pure-blooded Saiyans like the lack of hair growth is, as even clones with partial Saiyan genetic data have this ability as well. Examples are half-breed Saiyans, such as Gohan, and the biological menace Cell.
This ability has never been given an official name (other than being referred to as "Saiyan Power" in Battle of Z), however it is usually referred to as Zenkai (全開; lit. "Full Release") by the fandom.
After drinking the Ultra Divine Water and almost dying, Goku presumably got a strength upgrade from not just the water's powers, but also from a Zenkai. He also possibly got one after his near death encounter with King Piccolo. During the Namek and Frieza Sagas, Zenkai is an extremely important element of the series, which allows Goku and Vegeta's powers to increase in order to "catch up" to Frieza and his men. After the Frieza Saga, most of the Saiyans had already become Super Saiyans. In the Cell Saga, when Cell returned from his botched self-destruction on King Kai's planet, his power amplified to the point that high amounts of energy surrounded his body to mimic the aura of a Super Saiyan 2.
Zenkai's power seems to increase exponentially after each use. When Vegeta recovers from his first fight with Goku, his power level went from 18,000 to 24,000, and another was received after being nearly killed by Zarbon's transformed state and healed in a Medical Machine, which the Daizenshuu estimates to have augmented his strength to roughly 30,000. After Vegeta was again near-fatally injured after fighting Recoome, his strength strangely skyrockets enough to have an even power struggle with Frieza's first form (stated to be 530,000), and another, final Zenkai increased his power enough to see Frieza's movements and Death Beams at first, before he begins to put in even more effort. While the dub incorrectly implies that Zenkais only occur with Elite Saiyans, Goku, whom Vegeta and Nappa repeatedly refer to as low-level trash ("clown" in the dubbed anime), receives them many times. This is confirmed by how Krillin states that it was weird how Goku always seemed to grow ever stronger after recovering from fighting devastating battles against powerful foes, how Goku is able to learn and tap into the power of Zenkai's during his gravity training (which in total boosted his power from "over 8,000" to 90,000 in the Namek Saga), and how Goku's power again receives an enormous boost after recovering in a Medical Machine from his battle with Captain Ginyu (Goku's power level having increased from 90,000 to 3,000,000). To force a boost in power during the battle with Frieza, Vegeta tells Krillin to brutally injure him and have Dende heal him, which after greatly increased his power, though still not nearly enough to defeat the space tyrant.
Goku experiences a Zenkai upon eating a Senzu Bean in the movie Dragon Ball Z: Cooler's Revenge. In Dragon Ball Z: Broly - The Legendary Super Saiyan, it is stated that Broly survived his execution and grew stronger due to his Saiyan genes. In Dragon Ball Z: Broly - Second Coming, it is implied that Broly grew significantly stronger while in a coma for seven years after being beaten by Goku and recovering.
Video game appearances
In the RPG Dragon Ball Z: Super Saiya Densetsu, when a Saiyan reaches less than 5 HP, using a full heal item on him (like a card of Kami) will result in him receiving a Zenkai and raise his level by 1. This can be used early on against Raditz, to boost Goku's level extremely high. Also in the game, there is a glitch with Zenkai: if a player chooses to level Dende to level 3 (9999 exp earned twice), Dende can heal a character as well. Should he heal a Saiyan that is at 5 HP or below, the Saiyan receives the Zenkai, but not the level up. This allows certain characters like Gohan or Vegeta to be boosted to higher-than-usual power levels. Using the Moon Card on Gohan, however, restores his power level to the level it should be.
In Dragon Ball Z: Battle of Z, a customize card named "Saiyan Power" can be equipped onto a character and raises their attack power every time they are knocked out. This card is based on Zenkai.
In Dragon Ball: Xenoverse, Zenkai is one of the Saiyan race's unique traits. When a Saiyan falls in battle, and is revived, either through teammate assistance or a Battle Item, they gain a large power boost that lasts for the duration of that battle. A Zenkai effect can be applied to non-Saiyans through Z-Souls such as Vegeta's "Saiyans are a Warrior race!" or through Super Perfect Cell's "That was a handy miscalculation".
- This ability may be derived in principle from Friedrich Nietzsche's famous quote "Whatever doesn't kill me makes me stronger".
- The term Zenkai is used in the first Dragon Ball Z ending theme, "Detekoi Tobikiri Zenkai Power!"
- The Meta-Coolers' regeneration also corrects any flaw that allowed the unit to take damage, similar to Zenkai.
- In chapter 303, Vegeta states that a Saiyan cannot experience Zenkai if the wound is self-inflicted.