condom: a sheath of latex, nitrile, polyisoprene, polyurethane or natural skin (lambskin) that is worn over the penis to reduce the risk of pregnancy and/or sexually transmitted infection (STI) during oral, anal or vaginal intercourse. condoms, also known as external condoms, prevent pre-ejaculatory (pre-cum) fluid or semen from entering a partner’s mouth, anus or vagina. Only latex, nitrile, polyisoprene and polyurethane condoms prevent STI transmission. Condoms—including those made of natural skin (lambskin)—prevent pregnancy about 98 percent of the time when they are used correctly and consistently. Condoms should never be reused.