The Grey is the name given by the Isu Juno to the state of living inside an artificial paradise inside a digital network likely a resemblance to an artificial afterlife.  This is also the most common reference to her own status as an artificial intelligence.
Following her release from the confines of the Grand Temple by Desmond Miles on 21 December 2012, during his attempt to save the world from a solar flare, Juno entered the worldwide digital networks as an artificial intelligence. Using these networks, Juno became capable of interfacing and taking over various forms of digital technology, and has launched several attacks against Abstergo Industries and the Helix servers.
After her attempt to overtake the body of an Abstergo Entertainment research analyst with the help of the Sage John Standish had failed, Juno intended for humanity to "enter the Grey" by uploading people's digital consciousness through the Animus and live under her leadership. In a digital correspondence between Juno and her husband Aita decoded by Abstergo employees, she revealed that Clay Kaczmarek and Desmond Miles were among those who "will [not] leave" and "live in bliss" inside the Grey.