Ṣaḥīḥ
Ṣaḥīḥ, (صَحِيْح), is best translated as authentic. Ibn Hajar defines a hadith that is ṣaḥīḥ lithatihi, ṣaḥīḥ in and of itself, as a singular narration (ahaad – see below) conveyed by a trustworthy, completely competent person, either in his ability to memorize or to preserve what he wrote, with a muttaṣil (connected) isnād (chain of narration) that contains neither a serious concealed flaw (ʻillah) nor irregularity (shādhdh). He then defines a hadith that is ṣaḥīḥ ligharihi, ṣaḥīḥ due to external factors, as a hadith "with something, such as numerous chains of narration, strengthening it."
This definition of Ibn Hajar illustrates that there are five conditions to be met for a particular hadith to be considered ṣaḥīḥ. The first, is that each narrator in the chain of narration must be trustworthy. The second, each narrator must be reliable in his ability to preserve that narration – be it in his ability to memorize to the extent that he can recall it as he heard it, or, that he has written it as he heard it, and has preserved that written document unchanged. The third, the isnād must be connected, muttasil, in that each narrator could have at least conceivably heard from the previous narrator. The fourth, that the hadith, including its isnād be free of an 'illah or hidden, but detrimental, flaw – such as it being established that two narrators, while having been contemporaries, never, in fact, met thus causing a break in that 'chain'. And fifth, that that hadith be free of irregularity, meaning that it not contradict another hadith better established than it.