ما هو تعريف الحديث المتفق عليه؟

The "Agreed Upon Hadith" is what the two Sheikhs, Bukhari and Muslim, narrated. It may have been narrated through one Companion or more. It does not mean that the narrators agreed on its wording and meaning; rather, it may be transmitted with different wordings. The important thing is their agreement on narrating it and including it in their books, whether they agreed on its wording or differed therein. What matters is the agreement in meaning.