"Rock"

"But he turned and said to Peter, 'Get behind me, Satan!  You are a stumbling block to me; for you are setting your mind not on divine things but on human things."
~Matthew 16:23

"Because the way of Jesus to the cross became a σκάνδαλον* to Peter, he himself became σκάνδαλον to Jesus, i.e., a personified temptation to turn aside from God's will."

~Kittel, Gerhard, Gerhard Friedrich, eds. Theological Dictionary of the New Testament, vol. 7 (Grand Rapids: Eerdmans, 1964-1976), 348.

*Gk., a stone that trips someone or springs a trap to ensnare them