«وَخَبَرُ الْوَاحِدِ فِي اللُّغَةِ: مَا يَرْوِيهِ شَخْصٌ وَاحِدٌ، وَفِي الِاصْطِلَاحِ: مَا لَمْ يَجْمَعْ شُرُوطَ التَّوَاتُرِ»
Linguistically, khabar al-wahid is a report narrated by one person; technically, it is a report that does not fulfill the conditions of tawatur.
Linguistic wahid — a single narrator in the ordinary sense.
Technical ahad — the broad category opposed to mutawatir reports.
Thus a technically mashhur report with three independent routes is still ahad when tawatur is absent; an aziz report with two routes is also ahad. Restricting the technical term “khabar al-wahid” to reports that are literally narrated by only one person would be mistaken.