«وَهُوَ مَنْ لَمْ يَرْوِ عَنْهُ إِلَّا وَاحِدٌ وَلَوْ سُمِّيَ»
It is someone from whom only one person narrated, even if the transmitter’s name is known.
The qualification “even if named” matters. Knowing a person’s name does not imply the existence of multiple pupils reporting from that person. A specifically identified narrator can therefore belong to the wuḥdān category.
Muslim ibn al-Ḥajjāj and al-Ḥasan ibn Sufyān are among those credited with works concerning such narrators. Their writings document the historical patterns of transmission; the label does not assign one universal reliability grade to every person discussed.