[dragonwriter]

> My understanding is that “visum” means “that which has been seen” in Latin. What does “visa” mean then?

Basically the same thing, from the same root verb “vidēre”; vīsum is “that which has been seen” (noun), vīsa is “which has been seen” (adjective), from which English and some other languages have derived a noun “visa” as a shortening of the modern Latin “charta vīsa” (“paper/document which has been seen”) possibly through a french intermediary before English (different sources I’ve seen disagree on this.)