A Shadh (شاذ) Hadith is the differing of a Maqbool narrator with that narrator which is more Mawthooq/trustworthy than him. But as is found in some books on the Mustalah, it says that Shadh (الشاذ) is the differing of a Mawthooq/reliable/trustworthy narrator with other narrators which are also mawthooq (هو مخالفة الثقة للثقات). So what have scholars of hadith said in regards to this?
The word Shadh in Arabic means irregular, abnormal or unnatural.
What you stated is correct. Generally in hadith, Shadh is describing a hadith of a trustworthy person that contradicts the narration of a person who's more reliable.
This is what all scholars agree with. Some have added that it contradicts a narrator who's more reliable and has a better reputation. But this doesn't really make much of a difference.
If we have a Sahih hadith, all narrators thiqa and they reported a khabar that goes against other Sahih hadith, whose reporters are more reliable and they didn't mention it in a similar wording as the first. The former would be considered shadh.
So far I know, Shadh is more related to the narration itself and not the chain of narration. There may be some errors, even from the reliable narrators. Some words may get altered, while speaking or listening, thus altering meaning. Then the other similar narrations are compared and the defects are ruled out. The ruled out defective narrations are then called Shadh.