«وَالشَّاذُّ لُغَةً: الْمُنْفَرِدُ، وَاصْطِلَاحًا: مَا يُخَالِفُ فِيهِ الرَّاوِي مَنْ هُوَ أَرْجَحُ مِنْهُ»
Linguistically, shadh means isolated; technically it is a report in which a transmitter contradicts someone whose transmission is more preponderant.
Both contradiction and greater preponderance matter. The variation must be materially incompatible in context, and the competing report must be preferable on substantiated grounds. Greater weight may emerge from precision, memorization, agreement of routes, or other significant evidence.
Example: A trustworthy transmitter attributes a particular report to the wrong Companion while several more accurate students of the same teacher consistently preserve a different attribution. If careful comparison demonstrates the contradiction, the problematic route may be treated as shadh.