«وَالْمُعَلَّلُ لُغَةً: مَا فِيهِ عِلَّةٌ، وَاصْطِلَاحًا: مَا فِيهِ عِلَّةٌ خَفِيَّةٌ قَادِحَةٌ»
Linguistically, a muʿallal report has a defect; technically it has a subtle defect that genuinely impairs its authenticity.
Khafiyyah (hidden) means not immediately apparent on first inspection; qadihah (disqualifying) means that the defect truly undermines the report. A minor variation or an unexplained feature is not automatically a disqualifying ʿillah.
For example, comparison of several routes may show that one narrator presents a continuous chain while a better-preserved version is mursal, and the evidence indicates an error in the apparently continuous version. A chain that seems complete can then require reassessment. The central question is which version was actually preserved correctly.