'Sign' in the Bible

The Pharisees came out and began to argue [contentiously and debate] with Him, demanding from Him a sign from heaven, to test Him [because of their unbelief].

He groaned and sighed deeply in His spirit and said, “Why does this generation demand a sign? I assure you and most solemnly say to you, no sign will be given to this generation!”

“Tell us, when will these things happen, and what will be the sign when all these things are about to be fulfilled?”