Zehāb means ghayba (passing away from oneself, losing oneself). However, in terms of meaning, it is broader than the word ghayba. Zehāb is when the qalb (the heart), during the mushāhada (contemplative witnessing) of something, becomes distant from perceiving the maḥsūsāt (the world of sensory perception).
Afterwards, one becomes distant even from zehāb itself. (This) passing away from passing away (zehāb from zehāb, i.e., passing beyond self-effacement) has no end. (Lumaʿ, 423)
Zehāb is the qalb's, through the witnessing of the beloved—whatever that beloved may be—becoming absent from perceiving all that is felt (i.e., being absent from all that is sensed). (Iṣṭilāḥ, 534)
According to the people of Sufism (taṣawwuf), zehāb is the qalb's, through the witnessing of the beloved, becoming absent from perceiving all that is felt. For, O walī, for the ʿārif (gnostic), there is no possibility for the heart and the inner self (bāṭin) to remain. Just as for the lover, not even a single breath passes without the beloved, and he sees his beloved in everything with the eye of the heart, so the veils over his existence and senses prevent him, when he looks, from perceiving anything other than his beloved. He is thus veiled, and because of these veils, he desires to reach his beloved.
Just as the senses (perceptions) of a sleeping person disappear, so too does the ʿārif's perception of the sensory forms in the outward disappear, and the senses turn to khayāl (imagination); thus, he sees the image of his beloved in his imagination, draws it close to his heart, and ultimately sees it without any image. For imagination is not a degree between him and meaning, but only a means. Likewise, between him and the world of sensation, there is neither a means nor a degree. This is the means by which meaning descends to him and the perceived thing ascends to him. He, by his essence, accepts both sides.
When the ʿārif or the muḥibb (lover) transitions from the world of sensation (the world of perception) to the world of imagination, he draws near to the meaning of the Maḥbūb (the Beloved) and, in his imagination, witnesses it as an image of zehāʾirullāh.
While in the world of imagination, he sees himself. When he turns his gaze to the levels of meaning adjacent to the level of imagination, he encounters meaning face to face, without representation or form. Then, when he looks at the world of images and sensation, he realizes that when he conceives of this meaning existing in the world of sensation, all the forms that appear before him are its forms. Thus, the one who witnesses becomes absent with the witnessing of every maḥsūs (everything sensed, perceived by the senses), because they are not the forms of the Beloved; on the contrary, everything sensed is the form of the Beloved. The forms of the maḥsūsāt must depart from him, for they are not the forms of the Beloved. Now, he has come to witness Him in everything. This is zehāb. (Futūḥāt, II, 384)
(See also fanāʾ–baqāʾ; ghayba–ḥuḍūr)