Metanoia is the Greek word for repentance, and it literally means a change of mind — from meta (change) and nous (mind). But it is no mere mental adjustment: it describes a deep turning of the whole person, a reorientation of thought, will, and life away from sin and toward God. It was the opening word of both John the Baptist's and Jesus' preaching: repent, for the kingdom of heaven is at hand.
Metanoia is more than feeling sorry. Judas felt remorse and despaired; Peter repented and was restored. True metanoia turns around and walks a new direction — it is, as Paul says, godly sorrow that works repentance to salvation. And it is met with joy: heaven throws a party over one sinner that repenteth, for repentance is the doorway home.
Metanoia in Scripture
Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord.