Secondary sex characteristic are traits which give an individual an advantage over its rivals in courtship. They are contrasted with primary sex characteristics, which are usually directly involved in reproduction without playing a role in courtship (e.g., the uterus).
Secondary sex characteristic can give individuals an advantage over rivals in one of two ways. Either they can be used to defeat rivals in combat, or they can be used to attract members of the other sex. An example of the former is the antlers on deer; the male with the best antlers defeats his rivals, and thus gains access to the females of the group. An example of the latter is the peacock's tail; the male with the most impressive tail will woo more females than a less impressive male.
In humans, secondary sex characteristics include, in males, facial hair and deep voice, and in females, the breasts.