Medical Definition of horn
1a: one of the usually paired bony processes that arise from the head of many ungulates and that are found in some extinct mammals and reptiles; especially : one of the permanent paired hollow sheaths of keratin usually present in both sexes of cattle and their relatives that function chiefly for defense and arise from a bony core anchored to the skullb: the tough fibrous material consisting chiefly of keratin that covers or forms the horns of cattle and related animals, hooves, or other horny parts (as claws or nails)
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