What is repentance (tawba) in the Qur'an?

وَيَسْـَٔلُونَكَ عَنِ ٱلْمَحِيضِ ۖ قُلْ هُوَ أَذًى فَٱعْتَزِلُوا۟ ٱلنِّسَآءَ فِى ٱلْمَحِيضِ ۖ وَلَا تَقْرَبُوهُنَّ حَتَّىٰ يَطْهُرْنَ ۖ فَإِذَا تَطَهَّرْنَ فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ ٱللَّهُ ۚ إِنَّ ٱللَّهَ يُحِبُّ ٱلتَّوَّٰبِينَ وَيُحِبُّ ٱلْمُتَطَهِّرِينَ

They question thee (O Muhammad) concerning menstruation. Say: It is an illness, so let women alone at such times and go not in unto them till they are cleansed. And when they have purified themselves, then go in unto them as Allah hath enjoined upon you. Truly Allah loveth those who turn unto Him, and loveth those who have a care for cleanness.

إِنَّمَا ٱلتَّوْبَةُ عَلَى ٱللَّهِ لِلَّذِينَ يَعْمَلُونَ ٱلسُّوٓءَ بِجَهَـٰلَةٍ ثُمَّ يَتُوبُونَ مِن قَرِيبٍ فَأُو۟لَـٰٓئِكَ يَتُوبُ ٱللَّهُ عَلَيْهِمْ ۗ وَكَانَ ٱللَّهُ عَلِيمًا حَكِيمًا

Forgiveness is only incumbent on Allah toward those who do evil in ignorance (and) then turn quickly (in repentance) to Allah. These are they toward whom Allah relenteth. Allah is ever Knower, Wise.

وَلَيْسَتِ ٱلتَّوْبَةُ لِلَّذِينَ يَعْمَلُونَ ٱلسَّيِّـَٔاتِ حَتَّىٰٓ إِذَا حَضَرَ أَحَدَهُمُ ٱلْمَوْتُ قَالَ إِنِّى تُبْتُ ٱلْـَٔـٰنَ وَلَا ٱلَّذِينَ يَمُوتُونَ وَهُمْ كُفَّارٌ ۚ أُو۟لَـٰٓئِكَ أَعْتَدْنَا لَهُمْ عَذَابًا أَلِيمًا

The forgiveness is not for those who do ill-deeds until, when death attendeth upon one of them, he saith: Lo! I repent now; nor yet for those who die while they are disbelievers. For such We have prepared a painful doom.

إِلَّا مَن تَابَ وَءَامَنَ وَعَمِلَ عَمَلًا صَـٰلِحًا فَأُو۟لَـٰٓئِكَ يُبَدِّلُ ٱللَّهُ سَيِّـَٔاتِهِمْ حَسَنَـٰتٍ ۗ وَكَانَ ٱللَّهُ غَفُورًا رَّحِيمًا

Save him who repenteth and believeth and doth righteous work; as for such, Allah will change their evil deeds to good deeds. Allah is ever Forgiving, Merciful.

۞ قُلْ يَـٰعِبَادِىَ ٱلَّذِينَ أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ لَا تَقْنَطُوا۟ مِن رَّحْمَةِ ٱللَّهِ ۚ إِنَّ ٱللَّهَ يَغْفِرُ ٱلذُّنُوبَ جَمِيعًا ۚ إِنَّهُۥ هُوَ ٱلْغَفُورُ ٱلرَّحِيمُ

Say: O My slaves who have been prodigal to their own hurt! Despair not of the mercy of Allah, Who forgiveth all sins. Lo! He is the Forgiving, the Merciful.

يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ تُوبُوٓا۟ إِلَى ٱللَّهِ تَوْبَةً نَّصُوحًا عَسَىٰ رَبُّكُمْ أَن يُكَفِّرَ عَنكُمْ سَيِّـَٔاتِكُمْ وَيُدْخِلَكُمْ جَنَّـٰتٍ تَجْرِى مِن تَحْتِهَا ٱلْأَنْهَـٰرُ يَوْمَ لَا يُخْزِى ٱللَّهُ ٱلنَّبِىَّ وَٱلَّذِينَ ءَامَنُوا۟ مَعَهُۥ ۖ نُورُهُمْ يَسْعَىٰ بَيْنَ أَيْدِيهِمْ وَبِأَيْمَـٰنِهِمْ يَقُولُونَ رَبَّنَآ أَتْمِمْ لَنَا نُورَنَا وَٱغْفِرْ لَنَآ ۖ إِنَّكَ عَلَىٰ كُلِّ شَىْءٍ قَدِيرٌ

O ye who believe! Turn unto Allah in sincere repentance! It may be that your Lord will remit from you your evil deeds and bring you into Gardens underneath which rivers flow, on the day when Allah will not abase the Prophet and those who believe with him. Their light will run before them and on their right hands; they will say: Our Lord! Perfect our light for us, and forgive us! Lo! Thou art Able to do all things.