Adversary.

Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee. But he turned, and said unto Peter, Get thee behind me, SATAN: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men. ( Matthew 16:22-23 )

Dictionary of Words from the King James Bible. Public Domain. Copy freely.