The Lord Jesus, when speaking to His Jewish followers about those Galileans whose blood Pilate mingled with their sacrifices or the eighteen upon whom the tower of Siloam fell and they died, said "...except ye repent ye shall all likewise perish." (See Luke 13:1-5) Repentance is not optional with God! Apostle Paul told the men of Mars Hill after he had beheld their devotions to all the known gods and even one to the unknown God that that God, "now commandeth all men every where to repent..." (See Acts 17:30) Let me repeat, repentance is not optional with God.
So what is repentance? According to Strong's Hebrew Greek dictionary, the term repent, used by the Lord Jesus, found in Luke 13:3 and 5 is "metanoeō (pronounced met-an-o-eh'-o) From G3326 and G3539;" (Strong's numbers) and means "to think differently or afterwards, that is, reconsider (morally to feel compunction): - repent." Apostle Paul used the same Greek word in Acts 17:30. That gives a pretty well defined explanation. When one repents, he doesn't just make an outward show of change but he begins "to think differently or afterwards, that is, reconsider (morally to feel compunction)..."
The Bible says in Isaiah 55:6-9 "Seek ye the LORD while he may be found, call ye upon him while he is near: Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the LORD, and he will have mercy upon him; and to our God, for he will abundantly pardon. For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts." I am so glad that the Lord Jesus didn't think evil thoughts toward me when I was a sinner but granted me, "repentance unto life." (See Acts 11:18b and Jeremiah 29:11)
My definition of repentance then is to quit thinking your way and start thinking God's way. My friend, if you have never truly repented of your sins, do so today. God has promised you His mercy and an abundant pardon. Obey the gospel command of the Apostle Peter, "Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." (See Acts 2:38) God always keeps His word!