'Sign' in the Bible

And the Pharisees came forth, and began to question with him, seeking of him a sign from heaven, tempting him.

And he sighed deeply in his spirit, and saith, Why doth this generation seek after a sign? verily I say unto you, There shall no sign be given unto this generation.

Tell us, when shall these things be? and what shall be the sign when all these things shall be fulfilled?