Property
From wipipedia.org
Other than the usual uses of the words, to encompass a person's land, artifacts and general items of wealth, words like property and possession are used in Master/slave context to refer to the slave. This can be seen in phrases such as "He is everything to me and owns me completely; i am only his property."
To be property is to be enslaved. When the slavery is consensual, there are many people to whom this is a wonderful fantasy, an erotic thrill, or simply a comforting and fulfilling way of life.
Sometimes, slaves are trained to say "the Master" rather than "my Master", to avoid giving any suggestion that they might own their master in some way.