I think, personally, the defintion is based on intent and style, rather than gear choice, distance traveled, or nights slept. For instance, when I go out backpacking, I'm there to enjoy my surroundings and the companionship of others (or myself). However, when deployed on SAR missions, I would consider it "fastpacking". We have a specific goal (find, reach, extract a patient, etc), and the time needed to reach that goal is paramount.
I carry a much lighter load when "backpacking" as opposed to "fastpacking" on a SAR mission (helmet, work gloves, harness, steel rescue biners, rope, cord, webbing, and anchors really add up!), and the usage of the common equipment is different. When backpacking, I stop hiking at dusk, cook a hot meal and maybe some hot cider or hot chocolate, then sleep a full night, then wake up and cook breakfast before hitting the trail. When SAR "fastpacking", I may bivy for 2 or 3 hours if neccesary, make a hot drink if my core temp is getting low, and make food if the energy levels are really down, but all with the time constraints and final goal in mind. I do what is required to meet the goal, and nothing more. That's what constitutes fastpacking in my (meager, sleep deprived) mind.