After reading a few posts here today, I see that the definitions of "scratchbuilding" and "kitbashing" vary among some of you.
IMO, kitbashing means combining existing parts from different kits to create a model.
Scratchbuilding means creating components (or an entire model) from raw materials (sheet plastic or brass, structural plastic, brass or aluminum shapes, tubing or rod, wood, etc... not only modifying existing parts.
Of course, scratchbuilding and kitbashing can be combined.
Using two (or more) existing kits (and maybe a few parts box pieces) to create a model is an example of kitbashing.
Using raw materials (for example, the way Peter Lombardo carves his own body bucks and vacu-forms the body from sheet styrene) is an example of scratchbuilding.
Those are my definitions, FWIW.