Zill (shadow) is the existence of comfort behind the veil. (Iṣṭilāḥ, 539)
Zill is the relative existence (izāfī wujūd) that becomes manifest through the determinations (taʿayyun) of the possible essences (aʿyān-i mumkināt), which are comprised of non-existents (mādūmāt), and which are manifest (zāhir olan) (that is, which appear through the determinations of essences that are nothing but non-existence and are contingently brought into being). This is an existence that belongs to His Names and becomes manifest outwardly through the Name Nūr (Light). The darkness of its non-existence (ʿadamiyyat) veils the manifest light with its own forms. Thus, the manifestation of zill through light (nūr) and its being zill is due to its own non-existence. Allah says: "Have you not considered how your Lord extends the shadow?" (al-Furqān 25:45). That is, have you not seen how He spreads relative existence (izāfī wujūd) over contingent beings (mumkināt)?
Darkness (ẓulma), in contrast to this light (nūr), is non-existence (ʿadam). Every darkness is nothing but the absence of light, whose function is to radiate (illuminate). For this reason, kufr (disbelief) is called darkness because it is the absence of the light of iman (faith) in the human qalb (heart), whose function is to illuminate the heart. Allah says: "Allah is the ally of those who believe. He brings them out from darknesses into light." (Kāshānī, 6, 165; Jāmiʿ, 89)
(See also nūr-ẓulma)