Characteristic of species in which there are marked and obviously discernible physical and behavioral differences between the sexes.
For example, among lions, males have a distinctive mane and are physically larger than females.
Lions therefore are sexually dimorphic.
Similarly, in many bird species, the males have brightly-colored plumage and often other indicators, such as combs or unique feathering, while the females are relatively plain in appearance.
Among human beings, there are similarly readily-apparent differences.
Human males have broader shoulders and narrower hips than females; have a higher degree of muscularity and a lower degree of body fat; are, on average, taller and significantly stronger than females; have, again on average, coarser facial features and both facial and usually body hair as well; and of course do not have breasts, but do have external genitalia.
Human females, by contrast, have narrow shoulders and broad hips; a higher degree of body fat; are smaller and on the whole muscularly weaker than men; have finer facial features; are relatively free of facial and body hair; and have prominent breasts while lacking external genitalia.
This sexual dimorphism is so profound among human beings, that if we try, as a sort of thought experiment, to imagine men and women without these defining characteristics, we find that we cannot do it -- that our brains rebel at the notion of men and women who look exactly the same except for the presence or absence of external genitals.
In recent years, sociobiologists have made a persuasive case that human sexual bimorphism is not limited to appearance and reproductive apparatus, but extends to behavior as well.
We now know that cross-culturally, men are significantly more violent and aggressive than women; and also cross-culturally that men and women pursue different reproductive strategies, which account for many of the differences we see in male and female behavior.
For example, men universally judge the attractiveness of women based upon their youth and likelihood of bearing strong, healthy children; while women universally look for men who will be good providers for themselves and their children.
Among human beings then, as among many other species, sexual dimorphism isn't just physical, it's behavioral as well.
See also Natural Masculinity, Natural male sex aggression.