Premarital sex generally refers to sexual intercourse between two people who are not within the bond of a marriage. It is a term that is used largely among those who believe in abstinence until marriage. The term carries with it the implication that participants will later be married (whether to each other or different people) and are choosing to engage in sexual intercourse prior to that.
Sex prior to marriage used to be incredibly taboo, thus necessitating a specific term to refer to it (rather than simply calling it "sex"). In modern times the stigma around the activity has decreased significantly and, consequently, "premarital sex" has become something of an antiquated term. However, in many US schools sexual education is largely based in the idea of abstinence prior to marriage, keeping the idea of premarital sex as something worthy of its own label alive. Additionally, some people may choose to refrain from sex prior to marriage for religious reasons and, for these people, premarital sex remains noteworthy.
