What is the definition of repent?

The Word used in Greek is "metanoia". It means literally "to change mind". Compare this with the Word for sin "amarteia". Martyro means to witness or see. "a" before a Word negates it. So to be in "amarteia" is to "witness wrongly" or "see wrongly". Implying that repentance "metanoia" is a change of perception. One analogy commonly used in Patristic writings is that "amarteia" is "missing the mark" (imagine you are shooting an arrow and you aim badly, missing the target) metanoia (repentance) is adjusting your aim so as to hit the mark.