Like so many other sex definitions, there isn't a single agreed upon definition for casual sex. Which makes interpreting the wide range of research on the impact of casual sex somewhat daunting. Researchers have defined casual sex in different ways, depending in part on the purpose of their research and in part on their approach (e.g. psychology versus sociology versus nursing, etc...).
Despite variations one consistent element to most definitions of casual sex is that it is sex with someone you don't consider to be a spouse, boyfriend, girlfriend or partner, and that it is sex without any significant relationship commitments attached. Some researchers attach a time period to casual sex; labeling casual sex, for example, as sex with someone you have known for two weeks or less. But this seems problematic as in reality emotional connections may happen in a short period of time, and partners may be together for a long time and yet have no emotional connection.
As a sex educator who is interested mostly in individual experience and not in developing universal definitions, I would define casual sex simply as sex that takes place without any commitment beyond the sexual encounter. It might be with strangers, friends, or even old lovers. But the defining characteristic of casual sex, what makes it casual, is the separation of the sexual encounter from a sexual or intimate relationship.
