Someone can certainly control me under threat of violence (if not to myself, then to someone or something I cherish), or by deceiving me—but that only means I control my actions in accordance with their intentions. I can certainly lose control of myself in a rage, psychosis, or seizure, but in none of those cases do I relinquish my control. Someone can possess me by force, through systems like slavery, conscription, and compulsory labor, but do I possess myself the way I possess my computer? How could I possibly relinquish my body?
I think calling anarchism "self-ownership" lets Domination's foundation sneak inside the individual's skin—the institution of private property. Domination can't exist without holding a region of land by force, and before Domination began 6000 years ago, land was exclusively held in common. Self-ownership depicts individuals as sovereign rulers over their bodies and persons, internalizing the ruler-region master-subject relationship.
Further, private property enforced by the state is capitalism's foundation. Anarcho-capitalists may find it natural to define anarchism in terms of property, but the vast majority of anarchists—like myself—who believe capitalism is simply one kind of Domination certainly do not.
So the self-ownership definition assumes an impossible property relationship between oneself and oneself, and both it and the self-rule definition taint anarchism with internalized authoritarianism. I also see one final problem with these definitions: they regard anarchism as a condition enjoyed by the individual. Although individualist anarchism has a long tradition, I think it's better to acknowledge that Domination only affects entire communities, and all individuals are members of communities.
I'd just define anarchism as what the Greek word anarchos means: "without rulers." In my theory of political power, Domination is a tool for making tools of human beings, an engine that converts human energy into power and privilege for the rulers. If this engine didn't exist, no rulers could control and possess entire communities of subjects.
www.enginesofdomination.com