Pop music people seem to think it's anything romantic, dreamy, or just slow. I hold with the "song with story line" definition. Stuff like "Engine 143", "Barbara Allan", "The Devil Went Down To Georgia", "Stagolee", "Wild Bill Jones", "Three Wooden Crosses", "Ebony Eyes", etc. (trying to cover a variety of genres).

Choruses are OK but the song has to actually have a plot rather than just be about something. "the Jute Mill Song" and "Coal Tattoo" are about something but do not have plots, so I wouldn't call them ballads.