The art of fusion.....
Fusion is a very special ability where two warriors merge into one supreme being. There is an extreme great increase in powerlevel during the fusion period. In most cases the powerlevel of the fused being is equal to both powerlevels added together or even better. In that last case we can speak of a bonus. Fusion is often ignored in Dragonball Z and GT until there is really no other option.There are three kinds of fusion:
Fusion achieved by the fusion dance
Fusion achieved by the Potarra earrings.
This transformation is eternal and cannot be undone. Another great advantage is the fact that Nameks can fuse with endless other Nameks. The ritual consists of two Nameks fusioning into each other. Hereby the one places his hand on the chest of the other and absorbs him. The person who absorbs the other keeps control over the fusion. However, the resultating character of the fusion shows also characteristics from the absorbed person. This action greatly increases the powerlevel of the absorbing person.
Piccolo and Nail in the Frieza Saga, Dragonball Z (a.k.a. Piccolin, Piccolonail)
Piccolo and Kami in the Cell Games, Dragonball Z (a.k.a. Kamiccolo)
(Mostly the name Piccolo is still used after both of these fusions.)
This fusion is much better than the Fusion dance fusion, but it is definitive. Both characters put one earring at one of their ears: one at the right ear, the other at the left. When a strong cohesion force between both characters exists, both characters are being 'melted' together into one superior being.
Goku and Vegeta became Vegito in the Majin Buu Saga, Dragonball Z. Because Vegito was eaten by Buu, Vegeta and Goku split again in Buu's belly.
This action is performed like a kind of dance. Both characters run to each other, bending their bodies to each other. As their fingers touch, they become one. One mistake in the ritual of the fusion dance succeeds to create an awful mismade fusion being. This kind of fusion is not definitive, it is worn off within about an hour. After the fusion both characters must rest for half an hour to fuse again.
Goten and Trunks, both Super Saiyan 1, turned into Gotenks in the fusion saga, Dragonball Z. They could even reach Super Saiyan 3 in this merged form, but the fusion period was shortened for about half an hour.
Goku and Vegeta became Gogeta in movie 12 and in Dragonball GT. When Gogeta reached Super Saiyan 4, he was extreme powerful.