The fusion dance is a technique introduced by Goku during the Majin Buu Saga
To correctly perform the Fusion Dance, the fusees must create a perfectly symmetrical image of one another. The Fusion Dance is a short series of poses that are performed by two characters of equal power levels, roughly equal size and same race. In the series, the result of a correctly performed Fusion Dance is a superior being whose power is multiplied severalfold over that of the individual fusees. If one character performs the dance wrong, then it will result in an obese character who is much weaker than either fusee individually. If the timing of the poses is off, it results in a sickly looking fusion who is also much weaker than each fusee individually. Regardless, the fused character possess mixed physical traits of their original counterparts. If they synchronize their dance correctly, it results with a very powerful being.
Only one character in the original series has been seen as a result of the Fusion Dance; his name was Gotenks, being a fusion of the characters Goten and Trunks. Even though there was only one in the original series, one other fused character appeared in a Movie and in Dragon Ball GT. In Dragon Ball Z Movie 12, Gogeta is born from Goku and Vegeta, however he was first given the name Veku upon being incorrectly fused. The second time, he was correctly made and given the name Gogeta. Gogeta appears as a Super Saiyan 4 in Dragon Ball GT, and the very most powerful character in the Dragon Ball universe. The total fusion time lasts about 30 minutes, depending on the amount of ki that is used. After that, it takes the character's bodies another hour before they are able to fuse again.
The dance itself is explained by Goku in Dragon Ball Z Movie 12 as combining elements of both traditional fighting poses and water ballet. The performers must visualize two rivers flowing towards each other that get closer as they approach each other, then converge at two single points; the points collapse under the pressure and the two rivers flow into each other, becoming one. The movements right down to the fusees' breathing must be perfectly synchronized and symmetrical, or the fusion will fail. The fusees must also equalize their power levels.