«المشهور: هو ما كان من الآحاد في الأصل، ثم انتشر في القرن الثاني»
Mashhur is a report initially transmitted as ahad that subsequently became widespread in the second era.
The definition continues that it was then transmitted by groups unlikely to agree upon falsehood and received by the community with acceptance. This is a technical usage, more specific than simply saying a hadith is “popular.”
The footnote connects the early period with the Companions and the subsequent era with the Followers and their successors. Later popularity alone does not establish this particular usuli category.