2 Corinthians 2:10-11. Anyone whom you forgive, I also forgive. What I have forgiven, if I have forgiven anything, has been for your sake in the presence of Christ, so that we would not be outwitted by Satan, for we are not ignorant of his designs.
Satan is a Hebrew word that has been taken straight into the Greek and English, which is a pity because that obscures its meaning. It simply means ‘adversary’, and is translated that way ten times in the Old Testament. For example, 1 Kings 11:14, 23, 25 mention various adversaries of king Solomon.
Paul and the Apostles had to deal with various adversaries when they preached the Gospel. In Revelation 2:13, the Roman authorities are called Satan. In 1 Corinthians 7:5, Paul says that sexual desire can be a Satan.
In 2 Corinthians 2:11, Paul is referring to some members within the Corinthian Church as Satan because they had taken no action against one of their members who had been involved in gross immorality — 1 Corinthians 5. Now he rejoices that these adversaries had not succeeded in dividing the Church and the matter had been resolved.
Further information: For a longer more technical discussion of the reference to Satan a few verses after this, see How does Satan disguise himself as an angel of light?.