The correct hadith, as was defined, is that which having a consecutive chain of narrators from the beginning to the end, and transmitted through an accurate reliable narrator from his like being devoid of any oddity and defect.
In al-Taqrib al-Nawawi says: It (correct hadith) is the hadith whose isnad (chain of transmitters) is incessant through exact reliable narrators, without any oddity or defect. By saying it is sahih, it is meant: It is not definitely decisive, and when saying it is incorrect we mean that its isnad in not authentic, while the optional being that in regard of whose isnad it cannot be determined to be the most veracious of chains whatsoever. When saying: It is correct upon which or its veracity there is concurrence, they mean the concurrence of the two Shaykhs.
The most inclusive definition for it may be the following one: “It is the hadith that is reported in a self-assuring way, with immunity against oddity and defect.”
Al-Jurjani, in his Taâ€™rifat, writes: The sahih tradition is that whose wording being safe from poor ones, and meaning from any contradiction to a verse or a successive khabar or unanimity, with its narrator being reliable. Its opposite is the saqim (defective).
The degrees of a sahih tradition differ in respect of strength due to the dissimilarity of the characteristics requiring rectification. When they be meaningful out of the most likely supposition on which the veracity depends, this would necessitate their having degrees differing according to the strong points. If such be the case, whenever the hadith narrators be of high level of reliability and accuracy and other preference â€” obligating traits, this hadith would be more veracious than that whose narrators be on lower degree of reliability. At the time they made asanid differing from each other, they also classified the narrators into high and low ranks, giving priority to the Madinah narrators over the Basran ones, and making the Sham narrators lower than the Basran ones, and so on. There are several parts for the sahih traditions, that can be recognized through their books.
Adapted from: “Lights on the Muhammadan Sunnah” by: “Mahmud Ali Riyyah”