'Sign' in the Bible

and the Pharisees came forth, and began to dispute with him, seeking from him a sign from the heaven, tempting him;

and having sighed deeply in his spirit, he saith, 'Why doth this generation seek after a sign? Verily I say to you, no sign shall be given to this generation.'

Tell us when these things shall be? and what is the sign when all these may be about to be fulfilled?'