Isnād
إسنادThe chain of narrators that carries a hadith back to its origin.
Definition
Each narrator in the chain was studied for memory, honesty and whether he could actually have met the next link. An unbroken chain of reliable narrators makes a report ṣaḥīḥ; a break or a criticised narrator weakens it. This science of narrator biography (ʿilm al-rijāl) is what allows a report from the seventh century to be assessed today.