What is the name of the secret organization of Nobodies in Kingdom Hearts?

The secret organization of Nobodies in Kingdom Hearts is Organization XIII. It is introduced as a group of powerful Nobodies led by Xemnas, the Nobody of Xehanort.

The organization seeks Kingdom Hearts, a vast source of power created from collected hearts. Its members initially claim that they want to regain complete human hearts, since Nobodies were traditionally described as beings without hearts. Later revelations show that Xemnas has a larger plan: the group is meant to provide vessels for Xehanort's heart and help forge the χ-blade.

The name refers to the original group's thirteen positions, although the membership and purpose change across the series. Most members use names formed by rearranging their original human names and adding an X. For example, Lea becomes Axel and Ienzo becomes Zexion. A common mix-up is confusing Organization XIII with the Heartless or the Unversed: those are different kinds of beings and enemies in the Kingdom Hearts universe.