Possession

Possession was a term to describe someone or something having control over the body of another. While this normally had a negative connotation, not all possessions were the result of a malicious intent.

Anaphasic lifeform
Anaphasic lifeform

Chakotay
In 2371 Tuvok and Commander Chakotay were sent to explore a dark matter nebula. When they returned Chakotay was brain dead, but Chakotay's consciousness was actually floating around the ship. Commander Chakotay first possessed B'Elanna Torres in order to eject the warp core. He then possessed Captain Janeway in order to stop Tuvok and finally possessed Neelix to make a map. 

Ilari
An Ilari rebel named Tieran was able to possess Kes for a brief period in 2373. 

Komar
The Komar were non-corporeal beings that lived in a dark matter nebula, located in the Delta Quadrant. Chakotay and Tuvok encountered the Komar and Tuvok was possessed. The possessed Tuvok also tried to trick Captain Janeway into taking Voyager into the nebula in 2371. 

Organian
At least two Organian observers possessed members of the crew on Enterprisse NX-01 in 2154. 

Paxan
A paxan was able to possess Deanna Troi while the Enterprise-D was in orbit of their planet in 2367. 

Selected DNA exchange
An unknown lifeform was able to use selected DNA exchanges to possess the DNA of humanoid lifeforms in 2374. Several exchanges took place before it was captured. 

Trill
Some Trill completely take over the host body, a full possession unlike the "blending" seen in other Trill. The rite of zhian'tara overseen by a Guardian allows for the past lives of the symbiont to possess a volunteer host for a short time. The Rite of Emergence allows a host to bring forth the personality and memories of a previous host, so the two personalities can interact. 

Ux-Mal
Three disembodied criminals possessed members of the crew of the Enterprise-D on the moon Mab-Bu VI in an escape attempt in 2368. 

Wisp
Wisp. Crossing.