The Hebrew for “a mark” is (ʾôt sign), which has a semantic range that is very broad, can means “a sign,” (Gen. 1:14; 4:15), “a banner,” “standard,” or “flag” (Num. 2:2; Ps 74:4), as well as “a miracle, wonder, sign,” that is, a mighty act of God that serves as a message of wonder or to instill referential fear. (Ex. 4:8-9, 17; Nu 14:11, 22; Deut. 4:34; 6:22; Jos 24:17; Jdg 6:17; Neh. 9:10; 2Ki 20:8, 9; Ps. 74:9; 78:43; 105:27; 135:9; Isa 38:7; Jer. 32:20, 21) On this K. A. Mathews writes,
This “mark of Cain,” as it is popularly known, has proven to be a seedbed for confusion (v. 15b). “Mark” is the common word for “sign” (ʾōt); the exact nature of the sign or its place on the body (“on Cain”) is unknown. One Jewish tradition pointed to Cain himself as the “sign” who served to admonish others to repentance (Gen. Rab. 22.12). In effect this has become true for later generations, if not his own, for Cain the man has become a token of sin’s fruit and divine retribution (1 John 3:12; Jude 11). Although “sign” is used figuratively in several passages (e.g., Exod 13:9; Deut 6:8; 11:18), the only parallel is Ezek 9:4, where certain men receive a mark on the forehead. But even there it is in an extended vision in which it only has symbolic force. What is important here is its purpose: “so that no one who found him would kill him” (v. 15). “Mark” in our passage is not a sign of the “curse”; in fact, it assures Cain’s safety rather than acts as a reproach. The mark in Ezekiel’s vision had the same effect; it distinguished those who bore the brand and gave them protection.
The Bible does not say exactly what this mark was, but it is highly unlikely that it was a physical mark on his person. Such a mark would be meaningless centuries later when thousands of people were living before the flood. The sign was likely a verbal decree made by Jehovah to Adam and Eve, which would have become an oral tradition that would have been passed down from generation to generation, avoiding the murder of Cain for the sake of revenge. It would seem that at least one Jewish tradition was seeing this view similarly, i.e., it was not a physical mark. While this seems the preferred understanding, one cannot be dogmatic, it is simply inferred from what seems reasonable and logical, as well as in harmony with the use of the Hebrew.
 Ernst Jenni and Claus Westermann, Theological Lexicon of the Old Testament (Peabody, MA: Hendrickson Publishers, 1997), 67.
 K. A. Mathews, Genesis 1-11:26, vol. 1A, The New American Commentary (Nashville: Broadman & Holman Publishers, 1996), 278.