'Sign' in the Bible

See, I am waiting at the door and giving the sign; if my voice comes to any man's ears and he makes the door open, I will come in to him, and will take food with him and he with me.

And a great sign was seen in heaven: a A woman clothed with the sun, and with the moon under her feet, and on her head a crown of twelve stars.

And there was seen another sign in heaven; a great red dragon, having seven heads and ten horns, and on his heads seven crowns.

And I saw another sign in heaven, great and strange; seven angels having the seven last punishments, for in them the wrath of God is complete.