The author's wording: a hadith whose chain runs unbroken to its end, each narrator upright and precise taking it from another of the same quality, with neither anomaly nor hidden defect.
Two kinds: sahih in itself, and sahih by corroboration.
The author's wording: a hadith with an unbroken chain carried by an upright narrator whose precision falls somewhat short, the hadith being neither anomalous nor carrying a hidden defect.
Two kinds: hasan in itself, and hasan by corroboration.
A hadith that is hasan in its own right and also comes through a second route of equal or greater strength, in wording or in meaning, so it rises from hasan to sahih.
A hadith carrying weakness that is not severe: a narrator who is weak but not so weak that he drops below the rank still usable for support, or a mudallis who never stated plainly that he heard it, meaning he names his source while hiding that he did not take it from him directly, or a chain with a link missing. Two conditions must hold with it: the hadith is not anomalous, and it is narrated through another route like it or stronger, in wording or in meaning.
His example: al-Tirmidhi graded hasan a hadith on shortening prayer while travelling, narrated by Hajjaj ibn Arta'a and Atiyya al-Awfi, both faulted for frequent error though neither accused of lying, because a second route through Ibn Abi Layla carried the same report.
The author's preferred wording: the best way to define the weak hadith is that it lacks one of the conditions of the accepted hadith.
Reversing the definition of sahih gives the five conditions it lacks: a connected chain, upright narrators, precise narrators, not anomalous (shadh), and not defective (mu'all). A hadith is weak if it is missing even one.
Ibn Hajar's wording the author cites: the hadith narrated by one accused of lying, not known except from his route, and contradicting the established rules; above the weak and below the fabricated.
The invented, fabricated report falsely ascribed to the Messenger of Allah ﷺ, having no real connection to the Prophet; it is unlawful to narrate it knowingly except to expose it.
The scholars agreed it is unlawful for anyone who knows its state to narrate it in any sense, except joined to a statement that it is forged and a warning against it.