Dictionary.net

The mark made on the ear of a horse, cow, pig, or sheep by its owner ; and hence the token or signal by which a thing is known. So used also in the north of England. The laws of several of the states require the earmark of every proprietor to be recorded with the town clerk, as evidence for reclaiming strays, etc.- Bartlett's Americanisms.

n. A mark on the ear by which a sheep is known; — any distinguishing mark by which the ownership or relation of a thing is known.