The Major Sins are those acts which have been forbidden by Allah in the Qur'an and by His Messenger, may the choisest blessings of Allah and peace be upon him, in the Sunna (his practise), and which have been made clear by the actions of the first righteous generation of Muslims, the Companions of the Prophet.
Allah, Most High, says in His Glorious Book:
If you avoid the major (part) of what you have been forbidden (to do), We will cancel out for you your (other) evil deeds and will admit you (to Paradise) with a noble entry. (An-Nisa 4:31)
Muhammad Bin 'Uthman Adh-Dhahabi