You see, even if Jenova's body is dismembered, it will eventually become one again. That is what is meant by Jenova's Reunion.Hojo, Final Fantasy VII.
The Reunion Theory is a term in the Compilation of Final Fantasy VII. It is a scientific theory proposed by Professor Hojo positing that Jenova cells, once separated, will seek to reunite and return to the main body, influencing the minds of creatures they have been implanted in to do so.
I wasn't pursuing Sephiroth... I was being summoned by Sephiroth.Cloud Strife
Hojo put his Reunion Theory to the test in the aftermath of the Nibelheim Incident. The survivors of Nibelheim, including Cloud Strife and Zack Fair (and excluding Tifa Lockhart, who had been rescued before Shinra arrived), were rounded up and injected with Sephiroth cells, Sephiroth having merged with Jenova cells while still in the womb. The subjects were exposed to Mako in a procedure similar to the augmentation done to SOLDIER members.
As the Nibelheim survivors did not have the mental or physical strength to keep their sense of self, they became mindless puppets devoted to Jenova and Sephiroth called Sephiroth-clones. Zack, being a SOLDIER, resisted the effects, but Cloud succumbed and developed Mako poisoning. Hojo labeled the two as failures and held them in Shinra Manor for years, while the successful Sephiroth Clones were tattooed with numbers to keep track of them.
Five years after the destruction of Nibelheim, Sephiroth's body at the North Crater sends out a call for the Jenova Reunion. The main body of Jenova kept at Shinra Building awakens, escapes, and takes on Sephiroth's form. It attacks President Shinra, and in the chaos Cloud and the members of Avalanche imprisoned in the building are freed. Recognizing Sephiroth's Masamune near President Shinra's body, Cloud and Avalanche pursue the shapeshifted Jenova across the Planet to settle Cloud's grudge with him. Unbeknown to them, the reason Cloud is driven to chase Sephiroth is due to the call for the Reunion influencing his thoughts. At this time the other Sephiroth Clones begin to move, some seeking the Black Materia Sephiroth needs for his plans.
Cloud and the other Sephiroth-clones gather at the Whirlwind Maze, where the party witnesses Jenova in Sephiroth's form kill most of the Clones, sending them into the lifestream, and then transform into Jenova∙DEATH. The party defeats Jenova and reclaims the Black Materia. In an area abundant in crystallized Mako they meet Hojo, Rufus Shinra, and Scarlet. Hojo reveals his Reunion Theory, explaining he expected the Sephiroth Clones to meet at Midgar where Jenova's body was being held, but Sephiroth's will overrode the Reunion instinct and brought the Clones to where he was instead. Cloud, the last Clone, arrives at Sephiroth's body to deliver the Black Materia.
Geostigma and a legacy aren't enough. Not for a true Reunion.Kadaj
The Remnants of Sephiroth seek Jenova's remains, recovered and hidden by Shinra Electric Power Company. Kadaj tells Rufus Shinra the plague Geostigma that has infected the Planet is Jenova's legacy, and Vincent Valentine later tells Cloud it is caused by Sephiroth's cells corrupting the Lifestream and infecting the bodies of those who come into contact with the tainted Lifestream. In a sense, this makes them similar to the Sephiroth Clones.
The Remnants' goal is to find Jenova's cells to bring about Sephiroth's rebirth. As they are formed from spiritual energy, they are not complete copies of Sephiroth, and having no Jenova cells, he cannot exert control over them directly. The Remnants are guided by their instinct to find Jenova, believing her to be their mother, but without Jenova cells they have no Reunion instinct and cannot sense her.
Loz and Yazoo gather children infected with Geostigma from Edge, Denzel among them, and take them to the Forgotten City where Kadaj activates the Reunion instinct within them. The children lead the Remnants back to Edge, gathering around the Meteor Monument in the center of town, and the Remnants destroy it with Bahamut SIN, believing Jenova to be held inside the monument.
As Cloud and his allies battle Bahamut SIN, Rufus reveals to Kadaj he has been keeping Jenova's remains under his cloak, and in the confrontation Kadaj seizes the box and the Remnants escape into the Midgar ruins. As Kadaj battles Cloud, the box containing the cells is sliced open and Kadaj merges with the contents. Sephiroth exerts his power over Jenova's cells and shapeshifts Kadaj into a new body for himself, completing the second Reunion.
During production of Final Fantasy VII: Advent Children, Tetsuya Nomura was originally going to make "Reunion" the film's subtitle, but found it unfitting for Sephiroth's rebirth and decided on "Advent", the Latin word that refers to an impending arrival.
It is revealed that near the end of Final Fantasy VII, before AVALANCHE killed Hojo in Midgar, he uploaded a digital copy of his mind into the Worldwide Network. When Meteor almost struck the Planet, the power outage put the network off-line, and Hojo's digital copy was fragmented.
Three years later, the World Regenesis Organization reactivates the network, allowing Hojo's fragments to reform and take control of Weiss the Immaculate while he is engaged in a Synaptic Net Dive. Hojo tells Vincent he considers his digital reformation and control of Weiss a "neo-Reunion".
The mysterious dark-robed men with number tattoos wander around the Midgar slums, the locals telling Cloud they used to be in SOLDIER but now appear ill to the point of being unaware of their surroundings and unable to speak outside of repeating the word "reunion" in pained moans. Cloud Strife runs into numbers 49 and 2 in his adventures in the slums, and in Episode INTERmission, Yuffie Kisaragi meets numbers 11 and 16 in the outskirts of the Sector 7 slums. Sephiroth appears able to control the robed men, as he takes #2 over while in the Shinra Building to smuggle Jenova out.
Super Smash Bros. Ultimate
The time-limited Online Tourney "Dark Reunion", in which only fighters associated with darkness were playable, is presumably a reference to the Reunion Theory due to the key artwork prominently featuring Sephiroth and Midgar, as well as Meteor.