A longhorn is a cow or bull with very long horns, the kind that sweep out and up. The famous ones are Texas Longhorns from the United States — tough cattle, bred for dry land, with horns that can stretch wider than a person is tall. People also use longhorn for that breed itself, and you will see the word on steakhouse signs. Though actually, not every cow with long horns is that breed; the name is just the plain picture: long horns. (In another corner of English a longhorn can be a beetle with long feelers — but this entry is the cow.) Informal and farm-and-food English, not a classroom science word.