Alcoholism is pretty clear-cut. When you start drinking of a day, you massively overshoot; when you start drinking after a period of sobriety, it could be days, weeks, months, years, or even decades before your mind 'unfogs' and you're able to stop; and once you're sober, a drink will still sometimes seem like a good idea, even despite years of terrible consequences.
A relapse would be drinking again. It is safe to say that taking any other substances where your relationship with the substance is or could be like it is with alcohol is a relapse, too. Do addictions to certain drugs mirror alcoholism? Check the above definition of alcoholism against your experience of cocaine, opioids, benzodiazepines, etc., and there is clearly no doubt. There are other drugs (let's not list them) where there is no overshooting, where the vicious cycle does not develop, and where there is no real physical or psychological dependence; they're just fun and are not very good for you. Maybe the 'not good for you' includes triggering a relapse on a really dangerous drug, so perhaps best to avoid anything mood-altering altogether. Mapping the scheme of alcoholism onto all other mood-altering substances is unprincipled, however, because the analogy often breaks down rapidly.
What else is a relapse?
People often say that taking a mind-altering substance is a relapse. That should be clear, right? Well, unfortunately caffeine and nicotine pass the blood–brain barrier, as do certain nutrients, and lots of foods affect mood and mental status.
The definition is made murkier as soon as you realise that a lot of medication is psychotropic, and a lot of medication, even non-psychotropic medication, crosses the blood–brain barrier and causes changes in mood or mental status. Then there is the question of which mood-altering substances can produce an addictive cycle.
Legality is also terrible as a test, because what is lawful in one country is not lawful in another. It is a historical quirk that alcohol and nicotine are not illegal, whereas less addictive and less damaging drugs are not.
Even once you have drawn up your list of banned substances (which usually involves applying principles piecemeal, because few people would treat caffeine as a slip, even though it is clearly addictive and clearly mood-altering), you're not out of the woods: all sorts of activities that do not involve ingestion cause the release of chemicals that are mood-altering. Exercise, gambling, and worry are good examples. Is gambling a relapse? Probably depends on whether you're a gambler. Is worrying a relapse? I could construct a good argument for that being the case.
Never taken a mood-altering substance since your sobriety date? Good for you! Trouble is, you have. Your digestive system even produces tiny quantities of alcohol. You have likely drunk coffee, perhaps smoked, and certainly eaten sugar. Your body is also a chemistry set, constantly producing chemicals that alter your mood, and any analysis of the interaction between behaviour, chemical composition of the bloodstream, internal release of chemicals including neurotransmitters, and brain activity is extremely complex. How do you think gamblers get high? It's not by introducing a substance into the body; it's by using behaviour to alter mood, and, chemistry is certainly involved.
So, here's the conclusion: I don't drink. I don't take narcotics and I don't take medication that is mood-altering if I can help it, because I hate the effect now. There are lots of other things I don't do, too.
What I don't do is pronounce everything an addiction at the level of alcoholism or crack addiction, because it's not. What I also don't do is tell other people what to count as a relapse and what not to count as a relapse.