My definition is likely a good bit different from some others, and it is mostly based on the through-hikes I've made in the last few years.
For me: Hiking without specialized access to services one could not reasonably walk to along the trail corridor.
Maildrops COULD count as support if you have to have someone back home mail them out. For my AT thru-hike, I was supported in this aspect in that my mom took the pre-addressed maildrop (which I packed) to the post office and mailed it out with the cash I left for that purpose on the timetable I left her. Still reasonably self-sufficient, but lightly supported. Onthe Colorado, I mailed all my mail-drops out from Denver just before I started the trail. I wouldn't consider these as supported as I did the work myself.
The other types of support would be transport. If you're walking into town, you're not supported. If you're paying for a shuttle, you're supported. If you're hitching, this is a grey area for me, since you're somewhat "living off the land".
For most all of my longer hikes, I've had a degree of support (bymy definition), but minimally so compared to those who have friends or outfitters actually meet them at predetermined points.