It is a veil. (Hujwīrī, 474)
Rān (rust) is a veil between the qalb (the heart) and the sacred realm, arising from the domination of the nafs (the lower self) and the settling of corporeal, dark entities within it. This veil prevents the qalb from benefiting from the lights of rubūbiyya (divine lordship). Thus, the barrier between the qalb and the sacred realm is called rān. (Kāshānī, 147; Jāmiʿ, 83)
(See also: sadā)