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.
- Soul Linking: Possession via a high tech device created by Dr. Gero linking a natural or artificial soul to a person allowing the soul to take control of their body.
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.
Baby's ability to possess others appears as a CAA Special Ability called Infestation (寄生). Due to game mechanics, it is classified as a form of Fusion (as the ability is activated by fusing two cards).
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.
In Super Dragon Ball Heroes: World Mission, Baby's Infestation CAA Special Ability can be equipped to any non-Special type card using Accessories, allowing said card to fuse with another card in the same Custom Deck equipped with the same type of Infestation (the character they fuse into is determined by the Infestation variation).
In Dragon Ball FighterZ, Super Baby 2 is able to use this to summon the last member of the enemy team that he's defeated in battle. This not only gives him a third assist to make use of but essentially gives him access to all of that character's Assist moves at once.