What is the Islamic or Qur’anic definition of who is a martyr?
What are the criteria required to be fulfilled in order to count a person as a martyr for Allah?
In Quran the word شهید means Witness.
But in custom, we call any person who is killed in the way of Allah, Martyr. And as you know Imam Hussain (A.S.) and Hamzah(the prophet uncle) are the master of all the martyrs. This meaning has been clarified in Qur'an many times. But there is nothing about 72 virgins in heaven in none of them. Please mention that the way of Allah is not a way that everybody can claim about it. It is the straight way. It is the Prophet way. Have you ever heard that the Prophet has started a war against any Kafir? Have you ever heard that the prophet has killed a prisoner or slave or a captive from war? Have you ever heard that the prophet after winning the war, has killed the people in that city and took their wealth from them? So any Jihad is not okay and killing people by the name of Jihad and be killed by the name of Jihad won't let persons to enter heaven as a martyr. I bring you related the verses about this as below
وَلَا تَحْسَبَنَّ الَّذِينَ قُتِلُوا فِي سَبِيلِ اللَّـهِ
أَمْوَاتًا ۚ بَلْ أَحْيَاءٌ عِندَ رَبِّهِمْ يُرْزَقُونَ
Do not think of those slain for the cause of God as dead. They are alive with their Lord and receive sustenance from Him.
فَاسْتَجَابَ لَهُمْ رَبُّهُمْ أَنِّي لَا أُضِيعُ عَمَلَ عَامِلٍ مِّنكُم مِّن ذَكَرٍ أَوْ أُنثَىٰ ۖ بَعْضُكُم مِّن بَعْضٍ ۖ فَالَّذِينَ هَاجَرُوا وَأُخْرِجُوا مِن دِيَارِهِمْ وَأُوذُوا فِي سَبِيلِي وَقَاتَلُوا وَقُتِلُوا لَأُكَفِّرَنَّ عَنْهُمْ سَيِّئَاتِهِمْ وَلَأُدْخِلَنَّهُمْ جَنَّاتٍ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ ثَوَابًا مِّنْ عِندِ اللَّـهِ ۗ وَاللَّـهُ عِندَهُ حُسْنُ الثَّوَابِ
Their Lord answered their prayers saying, "I do not neglect anyone's labor whether the laborer be male or female. You are all related to one another. Those who migrated from Mecca, those who were expelled from their homes, those who were tortured for My cause, and those who fought and were killed for My cause will find their sins expiated by Me and I will admit them into the gardens wherein streams flow. It will be their reward from God Who grants the best rewards."
وَالَّذِينَ هَاجَرُوا فِي سَبِيلِ اللَّـهِ ثُمَّ قُتِلُوا أَوْ مَاتُوا لَيَرْزُقَنَّهُمُ اللَّـهُ رِزْقًا حَسَنًا ۚ وَإِنَّ اللَّـهَ لَهُوَ خَيْرُ الرَّازِقِينَ
Those who abandoned their homes for the cause of God and who then died or were murdered will receive honorable sustenance from God; He is the Most Generous and Munificent.
فَإِذَا لَقِيتُمُ الَّذِينَ كَفَرُوا فَضَرْبَ الرِّقَابِ حَتَّىٰ إِذَا أَثْخَنتُمُوهُمْ فَشُدُّوا الْوَثَاقَ فَإِمَّا مَنًّا بَعْدُ وَإِمَّا فِدَاءً حَتَّىٰ تَضَعَ الْحَرْبُ أَوْزَارَهَا ۚ ذَٰلِكَ وَلَوْ يَشَاءُ اللَّـهُ لَانتَصَرَ مِنْهُمْ وَلَـٰكِن لِّيَبْلُوَ بَعْضَكُم بِبَعْضٍ ۗ وَالَّذِينَ قُتِلُوا فِي سَبِيلِ اللَّـهِ فَلَن يُضِلَّ أَعْمَالَهُمْ
If you encounter the disbelievers in a battle, strike-off their heads. Take them as captives when they are defeated. Then you may set them free as a favor to them, with or without a ransom, when the battle is over. This is the Law. Had God wanted, He could have granted them (unbelievers) victory, but He wants to test you through each other. The deeds of those who are killed for the cause of God will never be without virtuous results.
وَلَا تَقُولُوا لِمَن يُقْتَلُ فِي سَبِيلِ اللَّـهِ أَمْوَاتٌ ۚ بَلْ أَحْيَاءٌ وَلَـٰكِن لَّا تَشْعُرُونَ
Do not consider those who are slain for the cause of God to be dead. They are alive but you are unaware of them.
الَّذِينَ آمَنُوا وَهَاجَرُوا وَجَاهَدُوا فِي سَبِيلِ اللَّـهِ بِأَمْوَالِهِمْ وَأَنفُسِهِمْ أَعْظَمُ دَرَجَةً عِندَ اللَّـهِ ۚ وَأُولَـٰئِكَ هُمُ الْفَائِزُونَ
To those who have believed in God, left their homes, and fought for His cause with their possessions and in person, God will grant high ranks and success
وَالَّذِينَ آمَنُوا مِن بَعْدُ وَهَاجَرُوا وَجَاهَدُوا مَعَكُمْ فَأُولَـٰئِكَ مِنكُمْ ۚ وَأُولُو الْأَرْحَامِ بَعْضُهُمْ أَوْلَىٰ بِبَعْضٍ فِي كِتَابِ اللَّـهِ ۗ إِنَّ اللَّـهَ بِكُلِّ شَيْءٍ عَلِيمٌ
Those who accepted the faith later, left their homes and strove with you for the cause of God are also your people. They relatives are nearer to each other according to the Book of God. God has knowledge of all things.
لَـٰكِنِ الرَّسُولُ وَالَّذِينَ آمَنُوا مَعَهُ جَاهَدُوا بِأَمْوَالِهِمْ وَأَنفُسِهِمْ ۚ وَأُولَـٰئِكَ لَهُمُ الْخَيْرَاتُ ۖ وَأُولَـٰئِكَ هُمُ الْمُفْلِحُونَ
But the Messenger of God and the believers with him fought for the cause of God with their possessions and in person and their reward will be all good things and everlasting happiness.
إِنَّمَا الْمُؤْمِنُونَ الَّذِينَ آمَنُوا بِاللَّـهِ وَرَسُولِهِ ثُمَّ لَمْ يَرْتَابُوا وَجَاهَدُوا بِأَمْوَالِهِمْ وَأَنفُسِهِمْ فِي سَبِيلِ اللَّـهِ ۚ أُولَـٰئِكَ هُمُ الصَّادِقُونَ
The believers are those who believe in God and His Messenger, who do not change their belief into doubt and who strive hard for the cause of God with their property and persons. They are the truthful ones".