In mythology, a demi-god is the child of a mortal human and a god. In Greek mythology, the gods (particularly Zeus) were said to have numerous affairs with mortals, resulting in the births of many demi-gods; some of the most well-known are Hercules, Theseus and Perseus. Greek demi-gods tended to live among men, but showed their divine origin through superhuman attributes such as strength, courage or (in the case of Orpheus) musical talent.
Some mythical beasts, e.g. Pegasus were the offspring of gods and other beings. While these could be considered demi-gods, as they are half-god, they are not usually categorized as such because they lack the necessary human lineage.
It should be noted that Jesus Christ is not considered a demi-god; rather, Christians believe that He is fully God and fully man at the same time. This matter was a source of much debate during the early centuries of the church, and some heresies may have contended that Christ was similar in nature to a demi-god; however, this is not a prevailing view in the present day.