"The brand of Cain"
Cain (standing) and Abel (kneeling)
Public DomainCain (standing) and Abel (kneeling)

The Old Testament tells the story of Cain and Abel in Genesis 4. Cain was the oldest of Adam and Eve's sons; Abel was the second born. Both sons brought offerings to the Lord; God "had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect" (verses 4 and 5). This angered Cain, and he murdered his brother. Later the Lord asked Cain where Abel was, and he responded with the oft-quoted question, "Am I my brother's keeper?" (verse 9).

As a result, God pronounced His punishment on Cain:

What hast thou done? the voice of thy brother's blood crieth unto me from the ground. And now art thou cursed from the earth, which hath opened her mouth to receive thy brother's blood from thy hand; When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth. And Cain said unto the LORD, My punishment is greater than I can bear. Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him. And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. (verses 10-16)

The Bible doesn't specify what the "mark of Cain" was. Some have speculated, though, that it would be some sort of mark on his face, perhaps on his forehead, which would readily identify the murderer to those who met him.