Psychological dependency is based on psychological distress caused by the drug when you're not intoxicated, physical dependency is when you crave the drug because your brain has receptors that have a chemical composition that reacts with the drug ure taking, because it mimics the existing chemicals in the brain, and after a certain time period, ure brain starts to produce less and less of the endogenous neurotransmitters, as the stimulation of the area of the brain caused by the reaction of the regular neurotransmitter with the bind site, is already being stimulated so the negative feedback (like supply and demand) is relaying back that less of this chemical needs to be produced, because this area of the brain is getting stimulation from another source, when you cut off the source (stop taking), then there are less neurotransmitters, less reactions and decreased activity in the brain, this can cause the user to feel drained, depressed and all those other psychological components, or if the area of the brain which is now suffering decreased activity, controls actual physical functions, such as vomiting, shitting then you will endure this, so from the brains perspective there's not much of a difference between habit forming (all experience(s) is governed by chemicals and there reaction with bindsites, and the conversion of chemical energy into electrostatic, or an action potential) and physical dependency, but physical dependency will usually cause the user to constantly use, because he will constantly feel ill, and anybody will tell you that drugs feel fantastic, so it will always come back to the psychological, cause the mind will take a fucking hell of a battering because not only do you not feel happy, but you don't feel physically capable of doing anything, so to answer your question, there really is no strict definition, the causes of both are very closely intertwined.