HADITH TERMINOLOGY
حسن
ḥasan
One step below sahih — the chain is sound and the narrators honest, but one of them was a slightly lighter memoriser. Still binding as evidence.
Hasan meets every condition of sahih except one: a narrator's precision (ḍabṭ) is lighter rather than complete. Ibn Ḥajar puts it as briefly as possible — "if the precision is light" (fa-in khaffa al-ḍabṭ).
This is not a "doubtful" category. Al-Nawawī states: "hasan is like sahih in being used as evidence, though beneath it in strength; which is why a group of scholars folded it into the category of sahih."
Two practical notes. First, "hasan al-isnād" is not the same as "hasan" — a chain can be sound while the text is not, due to anomaly or hidden defect. Second, al-Tirmidhī's "hasan sahih" is not a hybrid grade; Ibn al-Ṣalāḥ and al-Nawawī explain it as a hadith arriving by two chains, one meeting the sahih standard and one the hasan.
This is the most contested of the main grades. Al-Khaṭṭābī, al-Tirmidhī and Ibn al-Ṣalāḥ each give different definitions, and Ibn al-Ṣalāḥ's own two-part scheme is an attempt to reconcile incompatible earlier usages rather than a report of consensus. Ibn Ḥajar's tidier formulation does not always match classical usage either.