Nobody's perfect. It's just a fact of life. And when it comes to making significant life changes, imperfection is pretty much a guarantee. That means that "recovery" from any problem/addictive/compulsive behavior also tends to be imperfect. It is rare indeed for someone to make the decision to change their behavior, and to do so without any fits, starts, or backward glances.
No, it's much more likely that someone who decides to change their behavior will also struggle to do so, start and then pause, slip, backtrack, regroup and move on. In short, "lapses" back into an old behavior pattern are an expected part of recovery. As such, a lapse need not signify a relapse - a full return to the problem behavior, and abandonment of the recovery process.
What determines whether the reemergence of old behavior is a lapse or a relapse? As with many things, it largely comes down to how one thinks about it.
Have you decided, explicitly or implicitly, to give up on recovery? That's a relapse.
Are you trying to replace old behaviors with new ones, but the new ones are just not always strong enough to prevent the old ones? That's a lapse.
However, a lapse can (quite easily) become a relapse, for two reasons: (1) it reignites the habitual pattern (neural and behavior), and (2) many people experience a lapse, and then give up on recovery efforts, interpreting the lapse as an indication of failure and relapse.
The first reason is something that can be overcome, as long as people stay motivated to continue their recovery efforts. That means it all hinges on the second reason: people who see a lapse as a relapse no longer think there is any point in trying to resist the pattern, because they think it's already too late.
This tendency is part of the Abstinence Violation Effect. The concept of abstinence is black-and-white: you are either abstinent or you are not. If you slip, or experience a lapse, you are no longer abstinent. It's automatically counted as a failure, and equated with being back at square one. Think about 12-step programs, where people report their length of abstinence. An alcoholic who had a single drink after three years sober is counted as having the same length of sobriety as someone who is just getting sober for the first time. Often this AVE is enough to turn a lapse into a relapse. One drink becomes a bender, because "I already ruined it, I might as well go all the way."
I think it's absolutely crucial to relapse prevention to reframe slips and lapses. To invite curiosity about what can be learned from a slip to fine-tune the recovery plan. Or what areas of vulnerability might require additional coping strategies. As long as people stay hopeful, a lapse can actually strengthen one's ultimate recovery. We need to move away from black-and-white thinking when it comes to change, and make room for the messiness of the process. It's really the only chance for stable, long-term change.