The Nexus (aka The Nexus Of Joy) is a spatial anomaly that exists in the fictional universe of Star Trek and was key to the plot of the film Star Trek: Generations. It is a ribbon-like energy pattern that travels through space. When it comes into contact with sentient lifeforms they are turned into part of the energy pattern. There they exist in a dream-state where eternal happiness is achieved by the crafting of one's ideal universe to whatever he or she desires. It is very difficult to "snap out" of the false reality and return to one's original physical life once becoming one with The Nexus.

Captain Jean-Luc Picard merged with The Nexus, and rescued Captain James T. Kirk from within. Fans have theorized that as a result of Picard's entering the Nexus, everything that has happened to the Next Generation crew and universe in the movies that have followed have evolved around Picard, thus creating an alternate paradoxal existence, while the "real" timeline has gone on to reflect the "fact" that the Enterprise-D crew is now dead as a result of Viridian's destruction. Alternatively, the events in the movie can also be interpreted to mean that both Kirk and Picard did escape from The Nexus and rejoined the "reality" of Star Trek. This theorizing with regard to The Nexus about whether "reality" has been restored or whether to which degree it is altered, is similar to the various instances in Star Trek when, due to time travel, the timeline may have been altered permanently from a certain episode onwards. Often, the storylines of such episodes involve the phenomenon of the time travel paradox.