The state or condition of being frightened; the quality of being afraid or scared.
From 'frightened' plus the abstract noun suffix '-ness,' creating a noun that names the state or quality of being in a frightened condition.
Creating 'frightenedness' shows how English lets us turn any feeling into a noun—we could say 'she felt frightened' or 'her frightenedness was obvious,' though the first sounds much more natural.
Complete word intelligence in one call. Free tier — 50 lookups/day.