I believe that a general definition for a relapse would be something like a return to previous behavioral patterns, getting drunk/high and acting like an alcoholic/addict. It is necessary for some period of abstinence to allow for a 'return' to old behavior--if you have only one day sober it is probably not a relapse because there is no period of recovery to speak of--really what can you accomplish in one day towards recovery? A slip would be getting drunk/high but just a one-time thing, meaning you don't revert to old behaviors and continue with your recovery. Overall, this is all fairly semantic and different people will have different defintions, but I think the ones I am giving would be the same that professionals in addiction medicine would use. The point is just not to get drunk or high, regardless of whether it is relapse or slip or whatever you want to call it.