To know the definition of a sex addict is to understand that addiction as a class of disease has had a long history of controversy -- just what do we mean by addicted? This is just as true for sex addiction as any other. The old ideas were that to be a "real" addict, someone had to suffer physical withdrawal symptoms. The classic was alcohol addiction and Delirium Tremens, violent seizures and hallucinations.
Addiction as a mental event
A newer understanding of the definition of a sex addict, is a more modern idea that addiction can be entirely a mental event, with or without physical symptoms. In this view, the cravings, the stress and relief cycle, and the inability to stop an unwanted behavior are all hallmarks of addiction. So what is the definition of a sex addict?
Here the question for knowing the definition of a sex addict is, "When does a normal biological function, one that we are hardwired to enjoy, cross the line into an addiction?" The problem, for many scientists, is that sex is sought out and craved by almost everyone. That's normal. They lack a critical measure of when the behavior is just on the extreme and when it is actually addictive in nature.
Sex addiction in the DSM
For this reason, psychologists have dodged the issue of a definition of a sex addict. The DSM-IV by puts sex addiction in with another category, "Sexual Disorders Not Otherwise Specified," which includes, "distress about a pattern of repeated sexual relationships involving a succession of lovers who are experienced by the individual only as things to be used." This is different than hypersexuality, a condition that might be caused by mania or other mental disorders.
So what about using the term in the news or among the lay public? Well, the distinguishing characteristic is a loss of self control that leads to ruinous behavior. Is it "unwanted" in the psychological sense? Someone who engages in activity (masturbation, sex, stalking behaviors, pornography) that keeps them from living a normal life has a problem.
Regardless of whether we accept the label of sex addiction or not, there is something wrong. It makes sense to think about the definition of a sex addict in this way: A behavior that interferes with forming and keeping relationships and causes undue sacrifice or impairs functioning in society that causes feelings of remorse and requires concealing activities.
The good news is that the same techniques that help with other addictions also help these people. 12 step programs, group and individualized therapy, and pastoral counseling may all be of benefit.