["How young women define sexual intercourse has implications for their sexually transmitted infection (STI) risk. This study investigated young women's (1) definitions of sex, (2) understanding of a unique sex event, and (3) definitions of when a sex event begins and ends.", 'Using semi-structured interviews, young women were asked to define sex, define when a sex event began and ended, and were asked whether they thought their partners would agree. Interview transcripts were analyzed using thematic analysis.', 'Participants were recruited from an urban adolescent health clinic in the Northeastern United States.', 'Twenty-four heterosexual, sexually active young women contributed data for analysis.', 'None.', "Young women's definitions of sex varied. Some included anal and oral sex while others did not. Time between sex events, new condom use, and new erection were used to define unique sex events. Some believed sex began with foreplay. Others believed sex began when the penis entered the vagina. Some believed sex ended when the penis was withdrawn from the vagina. Others believed sex ended with orgasm for one or both partners. Young women talked about the influence of relationship type on their definitions of sex."] Is `` Sex n't something you do with someone you do n't care about '' : young women 's definitions of sex?