Probably from 'uwth (in the sense of appearing); a signal (literally or figuratively), as a flag, beacon, monument, omen, prodigy, evidence, etc. -- mark, miracle, (en-)sign, token.
KJV: And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
NASB: Then God said, "Let there be lights in the expanse of the heavens to separate the day from the night, and let them be for signs and for seasons and for days and years;
KJV: 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.
NASB: So the LORD said to him, "Therefore whoever kills Cain, vengeance will be taken on him sevenfold." And the LORD appointed a sign for Cain, so that no one finding him would slay him.
KJV: And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations:
NASB: God said, "This is the sign of the covenant which I am making between Me and you and every living creature that is with you, for all successive generations;