Possession is a technique used by certain characters, and a form of fusion.
The technique involves one being forcibly taking over the body of another, acquiring their power and abilities in the process.
During the Piccolo Jr. Saga, Kami possessed the body of a man named Shen through mystical means, intending to defeat Piccolo Jr. under the public eye and trap him using the Evil Containment Wave. He left Shen's body when Piccolo reversed the technique to avoid having Shen trapped with him, not wanting him to suffer for his failure.
Watagash is a parasitic criminal capable of taking control of his hosts, lured by the darkness in people's hearts. It is then capable of powering them up; the more evil they are the greater the power up. When possessing a host with a substantial amount of darkness in their heart, Watagash is capable of causing them to undergo at least two different transformations. 
Baby's main strength comes from his parasitic ability. Over the course of the Baby Saga, he possesses several individuals, including Trunks, Gohan, and Goten, before settling on Vegeta as his prime host. The Tuffle Parasites also use parasitism to control their hosts.
- Parasitism: Possession via taking control of someone biologically, this is the version used by Watagash, Oren, Kamin and Tuffle Parasites.
- Tuffleization: The post-teen Tuffle parasite ability to convert a host into a Tuffle hybrid under their control.
Video Game Appearances
In Dragon Ball Z: Shin Budokai, Janemba is able to possess those who take in too much evil energy, and he does so to Vegeta and seemingly Frieza; the fake Majin Vegeta created by Janemba does the same to Vegeta.
In Dragon Ball Heroes, Baby is capable of possessing both Janemba and Hatchiyack, creating Baby Janemba and Baby Hatchiyack respectively. In one mission, he chooses to take Trunks as a permanent host rather than Vegeta.
In Dragon Ball Fusions, Baby is capable of possessing Hirudegarn who due to his naturally giant size is the perfect host for Adult Baby, as Hirudegarn's size is comparable to Great Ape Baby Vegeta thus Adult Baby can possess him easily without worrying about Hirudegarn becoming smaller like Vegeta did after his tail was cut off as Hirudegarn's forms are both gigantic sized. However Baby is forced to abandon Hirudegarn's body after Baby Hirudegarn is defeated by Tekka's Team.