It is a ḥadīth in which a hidden defect (ʿilla) damaging to its soundness has been detected, even though it outwardly appears free of any such defect.
Recognising the ʿilla is helped by noting the narrator's solitary transmission, or his being contradicted by someone else, together with circumstantial indications that alert one familiar with this art to: an irsāl in what is apparently mawṣūl; a waqf in what is apparently marfūʿ; the intrusion of one ḥadīth into another; or some other lapse on the narrator's part, to the point where the suspicion of such a defect predominates.
One then suspends judgement on the ḥadīth, since each of these factors is a barrier to declaring sound any ḥadīth in which it is found.1
Muqaddimat Ibn al-Ṣalāḥ, p. 71. The difference between al-shādhdh and al-muʿallal: al-shādhdh is far subtler than al-muʿallal, since in al-muʿallal the defect can be identified, whereas in al-shādhdh it cannot be; the critic senses that the ḥadīth is in error but cannot produce a definitive argument for it. ↩
When a narrator's solitary transmission is suspected, others' parallel narrations are mutābaʿa or shāhid; the pursuit of these is al-iʿtibār.
Al-mudraj is the mixing of what belongs to one speaker with what belongs to another without indication. It falls into mudraj al-matn (more common) and mudraj al-sanad.
A ḥadīth in which a narrator has been replaced with another, whether one narrator, several, or even the entire chain, is al-maqlūb (literally, that which has been inverted).
Al-mawḍūʿ is that which has been concocted and fabricated. It is known by four indications, and may not be narrated to one who knows its condition without disclosure.
