A ribbon of energy that crosses the galaxy every 39.1 years and opens onto a timeless realm of perfect contentment, where James Kirk was trapped for 78 years and which Tolian Soran killed to re-enter.
The Nexus drives the plot of Star Trek Generations. From outside it appears as a violent energy ribbon that destroys ships in its path. Anyone swept inside finds a reality shaped by their own wishes, where time has no meaning and they may go anywhere, at any moment. Guinan describes it as being wrapped in joy.
In 2293 the ribbon wrecks two ships carrying El-Aurian refugees. The Enterprise-B rescues some of them, among them Soran and Guinan, but Kirk is pulled into the Nexus when the hull is breached and is believed dead.
In 2371 Soran destroys the Amargosa star, and plans to do the same in the Veridian system, to bend the ribbon's course to a planet where he can meet it. Picard is carried in with him, finds an echo of Guinan and persuades Kirk to leave. Together they stop Soran on Veridian III, at the cost of Kirk's life.