Any part of one's anatomy which has any sexual importance, generally by being a differentiating characteristic between the two most common sexes. In general, the obvious bits (penis, vulva, vagina, uterus, testicles, etc.) are primary, some less-direct ones (nipples, mammary glands, cowper's/prostate) are secondary; it could be argued that some other minor differences (the shape of the pelvis and cheekbones) are sexual characteristics as well.