Whether a thing can actually be used by the bodies and minds that meet it. Accessibility is a property of the design, never of the user.
Medieval Latin accessibilis (approachable), from Latin accedere (to approach); the noun names a property of the design that is met, not a limitation of the one meeting it.
Whether a design can actually be met by the bodies and minds that come to it, a property of the making and never of the one meeting it.