[color=003399]Romans 2 NKJV
Or do you despise the riches of His goodness, forbearance, and longsuffering, [i][b]not knowing that [u]the goodness of God leads you to repentance[/u]?[/i][/b]
But in accordance with your hardness and your impenitent heart you are treasuring up for yourself wrath in the day of wrath and revelation of the righteous judgment of God, who "will render to each one according to his deeds": eternal life to those who by patient continuance in doing good seek for glory, honor, and immortality; [u]but to those who are self-seeking and do not obey the truth, but obey unrighteousness[/u] -- indignation and wrath, tribulation and anguish, on every soul of man who does evil, of the Jew first and also of the Greek; ... vv 4 - 9a
Matthew 3:7
But when [John the Baptist] saw many of the Pharisees and Sadducees come to his baptism, he said unto them, O generation of vipers, who hath warned you to [u]flee from the wrath to come[/u]?
Acts 17 KJV
And the times of this ignorance God winked at; [u]but now commandeth all men every where to repent[/u]: because he hath appointed a day, in the which he will judge the world in righteousness by [that] man whom he hath ordained; [whereof] he hath given assurance unto all [men], in that he hath raised him from the dead.
1 Thess 1:2 - 10
We give thanks to God always for you all, making mention of you in our prayers; remembering without ceasing your work of faith, and labour of love, and patience of hope in our Lord Jesus Christ, in the sight of God and our Father; knowing, brethren beloved, your election of God.
For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance; as ye know what manner of men we were among you for your sake. And ye became followers of us, and of the Lord, [u]having received the word[/u] in much affliction, with joy of the Holy Ghost: so that ye were ensamples to all that believe in Macedonia and Achaia.
For from you sounded out the word of the Lord not only in Macedonia and Achaia, but also in every place your faith to God-ward is spread abroad; so that we need not to speak any thing. For they themselves shew of us what manner of entering in we had unto you, and how [u]ye turned to God from idols[/u] to serve the living and true God; and to wait for his Son from heaven, whom he raised from the dead, [even] [b]Jesus, which delivered us from the wrath to come[/b].
Acts 2
22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know: [b]Him, being delivered by the determinate counsel and foreknowledge of God[/b], ye have taken, and by wicked hands have crucified and slain: whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it...
36 Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ. Now when they heard [this], they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men [and] brethren, what shall we do?
Then Peter said unto them, [u]Repent[/b], 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. For the promise is unto you, and to your children, and to all that are afar off, [even] as many as the Lord our God shall call. And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation.
Then [u]they that gladly received his word[/u] were baptized: and the same day there were added [unto them] about three thousand souls. And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers. [/color]
In a nutshell, if there is sin in your life past or present, from which you have [i][b]not turned[/i][/b], then true repentance will enable you to feel both clean from your past, and empowered to cease that/those sin(s)from that moment on.
Then, you'll know you have repented, because you'll have the fruit of repentance in your life.
That's quite difficult to define, but don't forget restitution if it's relevant to your relationship with other people (which would mean it is relevant to God, since it's His idea in the first place), or, major changes in the way your life looks to observers, as a testimony to them that God has changed [u]your heart[/u].
[u]Eternal[/u] life is a free gift, but, we have been freed from sin (through repentance and faith and whatever healing/salvation is necessary) to walk in newness of life - righteousness, truth, and good works - while we're still on this earth. This is where eternal life begins.
[color=003399]Luke 24
45 Then opened he their understanding, that they might understand the scriptures, and said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day: [u]and that repentance and remission of sins should be preached in his name[/u] among all nations, beginning at Jerusalem.[/color]
Remember... when Paul was writing to Israel indirectly through his letter to the Romans, he was writing to a nation who had had [i][b]only repentance[/i][/b] preached to them by both John the Baptist and Jesus. This was a new message for Jews. They had never before been asked to [i]care[/i] about their sins enough to stop sinning, and still it was impossible, unless Jesus had spoken to them, healed them or delivered them. All that the law had done was bring sin to their attention.
Those in Israel who were watching for their Messiah were ready in their hearts. If He said 'repent' - they repented!
Thus, when Paul writes in Romans 10, he doesn't even mention repentance, but notice, Peter put it [u]first[/u] to the Jews present, who had come from other countries, on the day of Pentecost. But, the resurrection of Jesus Christ had eclipsed every other sign from God, beyond all imagination. A MAN had risen from death without any other intermediary, and been seen by over 500 people at once. As Jesus had said to James 'Thomas, because thou hast seen me, thou hast believed: blessed [are] they that have not seen, and [yet] have believed.
John continues 20:30 And many other signs truly did Jesus in the presence of his disciples, which are not written in this book: but these are written, [u]that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name[/u]. (1 Peter 1:8)
Resurrection was a key Jewish belief, but everyone had thought it wouldn't happen until after they had all died physically, like what Martha said to Jesus in John 11.
But, [u]we[/u] [i]must[/i] accept the sentence of death upon [u]sin[/u] our lives. Repentance is the natural outflowing of our individual realisation that we are worthy only of eternal death because of sin, but Christ has saved us from [i]that[/i] death.
In response to repentance and faith, He can give the Holy Spirit to enable us to live free from sin - as others have explained earlier in the thread.
Repentance and faith are both gifts from God, and if we don't feel any of either of them, just as James said 'if any man lack wisdom, let him ask' we too, can ask our gracious God for repentance and faith, or, faith and repentance. He will understand the seeking of an honest heart, and He will reward it appropriately. Heb 11:6.
I hope this post doesn't seem like more woffle. I suspect if it does, that you may need to take some serious time out of your normal routine - even just one day - to focus on God and lie face down in His presence until you hear what you need to hear from Him.
Regarding a definition of repentance, a word search on it in the Old Testament brings up 45 entries in the KJV, which may give you some direction.
There is also this message in which Ron Bailey traces what we can understand by 'repentance' based on what we find in scripture only.
[url=http://mp3.biblebase.com/download.php?file=19]Repentance[/url]