A relapse can be said to be a setback after a period of progress where one reverts to a previous state. When speaking in terms of drug abuse and recovery, it is a return to the drug of choice after a period of living without that drug in an effort to rid oneself of the habit. The sad truth about relapse is that for many people they will go through cycles that involve relapse for the rest of their lives, but just as many manage to live without their drug of choice for good. It very much depends on the individual. Within one year about 1/4th or 25% of individuals will return to cocaine use on a weekly basis.
Medical professionals have stressed that relapse is not a binary state where you are either sober or not sober. Rather they describe relapse as a progressive decline in which the state of sobriety deteriorates giving way to the previous lifestyle and eroding the progress made bit by bit. The notion that it is binary contributes to further relapse by convincing the recovering addict that they have succumbed completely and totally to the addiction when this is in fact not the case. Recovery is not zero sum, all or nothing. It is a process.