The study from the Kinsey Institute at Indiana University found that no uniform consensus existed when a representative sample of 18- to 96-year-olds was asked what the term meant to them.
Around 30 percent of the study participants did not consider oral sex as sex, while for 20 percent of the participants anal sex was not sex either.
A surprising number of older men did not consider penile-vaginal intercourse to be sex.
More than idle gossip, the answers to questions about sex can inform -- or misinform -- research, medical advice and health education efforts.
"Researchers, doctors, parents, sex educators should all be very careful and not assume that their own definition of sex is shared by the person they're talking to, be it a patient, a student, a child or study participant," said Brandon Hill, research associate at the Kinsey Institute.
The study, conducted in conjunction with the Rural Center for AIDS/STD Prevention in IU's School of Health, Physical Education and Recreation, delves deeper into a question first examined in 1999 -- in the midst of a presidential sex scandal where the definition of sex was an issue.
Researchers from The Kinsey Institute asked college students what "had sex" meant to them, taking the approach, which was unique then, of polling the students on specific behaviours. No consensus was found then, either.
The new study examined whether more information helped clarify matters- study participants were asked about specific sexual behaviors and such qualifiers as whether orgasm was reached-and researchers also wanted to involve a more representative audience, not just college students.
"Throwing the net wider, with a more representative sample, only made it more confusing and complicated. People were even less consistent across the board," said Hill.
The study involved responses from 486 Indiana residents who took part in a telephone survey conducted by the Center for Survey Research at IU.
Participants, mostly heterosexual, were asked, "Would you say you 'had sex' with someone if the most intimate behaviour you engaged in was ...," followed by 14 behaviourally specific items.
Responses did not differ significantly overall for men and women. The study involved 204 men and 282 women.
95 percent of respondents would consider penile-vaginal intercourse (PVI) having had sex, but this rate drops to 89 percent if there is no ejaculation.
81 percent considered penile-anal intercourse having had sex, with the rate dropping to 77 percent for men in the youngest age group (18-29), 50 percent for men in the oldest age group (65 and up) and 67 percent for women in the oldest age group.
71 percent and 73 percent considered oral contact with a partner's genitals (OG), either performing or receiving, as having had sex.
Men in the youngest and oldest age groups were less likely to answer "yes" compared with the middle two age groups for when they performed OG.
Significantly fewer men in the oldest age group answered "yes" for PVI (77 percent).
The study has been published in the international health journal Sexual Health.