For me, groundwork is working in a halter and leadrope on the basics of leading, learning to disengage and yield certain parts of the horse, helping the horse overcome a fear or "mental-obstacle" via desensitizing and sacking out.
Lunging can be part of ground work but I consider that separate. And that's mainly a substitute for riding. Riding the horse has the rider to worry about, lunging the horse can work on its own gaits and tempo and get exercise. I like to keep those sessions short and sweet.