Synch Nexus

"Finish what you started, until your ancestor has nothing left to show you. That is a synch nexus."

- Subject 16 to Desmond

A Synch Nexus was a moment where an ancestor's memories came to a conclusion, and there was nothing left for their descendant to see or experience.

In the year 2012, Desmond Miles fell into a coma after stabbing his fellow Assassin, Lucy Stillman, under the influence of Juno. Upon being plugged into the Animus and being sent to the Black Room, Desmond met the visual representation of Subject 16. Subject 16 eventually told Desmond that the only way to escape from the Animus was to achieve a snych nexus, the complete conclusion of his ancestor's memories.

Desmond then began to relive the later memories of his ancestor Ezio Auditore da Firenze, as he searched for the Masyaf Keys that would allow him to relive key moments in Altaïr Ibn-La'Ahad's life.

Completing the memories of Ezio and Altaïr, Desmond achieved a synch nexus and was pulled into the Nexus of Time. Once there, Jupiter elaborated on the location of the Grand Temple and the First Disaster. He also charged Desmond with going to the Grand Temple.

After this, Desmond was sent back to the Black Room. The Animus had scheduled Desmond and Subject 16 for deletion, however, at the last moment, 16 pushed Desmond out of harms way. Waking up from his coma in front of Rebecca, Shaun, and his father, William Miles, Desmond told them that he knew where to go.

Source

 * Assassin's Creed: Revelations