According to Merriam-Webster, relapse is defined as
the act or an instance of backsliding, worsening, or subsiding
a recurrence of symptoms of a disease after a period of improvement.
In terms of recovery, this is a perfect description of the sequence that relapse invariably follows: backsliding, or a general negligence in regard to maintaining the habits of mind and action necessary to stay sober, leads to a recurrence of drinking or drug use. Clearly this could be described as a symptom of a disease after a period of improvement.
Is it really just a slip?
In recovery circles, relapse is often called a "slip," although the term is misleading, as it seems to describe a random, unpredictable event that may even have an external cause.
We slip on a patch of ice. We have a slip of the tongue.
The fact is, most relapses don't qualify as "slips" in this sense, as they are not only predictable and avoidable but also traceable to behavior and attitudes weeks and even months prior to the actual event.
If a diabetic fails to eat carefully and neglects his medication, the ensuing blood-sugar event wouldn't qualify as a slip, but the failure to follow medical advice would follow a familiar pattern: I'm feeling better now, so this whole thing isn't that big of a deal after all.
The biggest trap for people in recovery is complacency—the idea, after a few weeks, months, or even years of sobriety, that the original problem wasn't that bad, I've got a handle on it now, and that the conventional wisdom in recovery circles (stay involved, stay active) is burdensome and not really necessary.
For a real alcoholic or drug addict, relapse is usually just the beginning of a further descent to a new and more painful bottom, greater personal loss, and more harm inflicted on others.
When an addict is fully convinced of the inevitability of this, he or she has a good chance of long-term recovery, if there is an ongoing willingness to take responsibility for one's own sobriety.