addiction is a term used to describe the dependant effect of a chemical on neural plasticity and how the neural pathways are structured (how the webs of synapses are layered) as a cause of regularly altering neurochemistry,, i dont see how else it can be defined,,
Your definition cuts right to the heart of the matter. Is this chemical the substance ingested ? ... or could it be a chemical internally produced as part of our reward system? I guess both endogenous and exogenous chemicals fit within this framework.