One of the key concepts of IE is the literal slave: this is just the everyday definition of "slave" and it doesn't include the roleplay slaves you sometimes meet elsewhere in BDSM. For example, the second edition of the Oxford English Dictionary gives: "One who is the property of, and entirely subject to another person, whether by capture, purchase or birth; a servant completely divested of freedom and personal rights." When we say "slave", we mean literal slave, defined in this way. (Contrast with Ongoing Voluntary Submission.) Also used as a level of experience, to signify that someone has lived in this condition, in the four-point experience scale: none, novice, servant and slave.