The feature of copyrights that makes it possible for one rightsholder to transfer ownership of the rights to another person.
One of the basic characteristics of property is that it can be transferred to others, whether by sale, gift, or something else. Copyrights are no different. While a creator initially holds copyright in his or her creative work, those rights can be transferred to another person or entity, who can then transfer them again, etc. For example, recording artists frequently transfer the copyright in their songs to a records company. Michael Jackson famously owned the rights to all of the Beatles’ music.
The various rights that “copyright” subsumes can be transferred as a block, but more often are transferred or sold one at a time, some times to different people.