Question Summary:
What is the definition of a major sin?

Question Detail:

What defines a major sin [Kaba'ir] in the Usul of the Hanafi's? - Could you please elaborate on this. 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
In Sharhul Aqaaid, Imam Shamsuddin Taftazani, rahmatullahu alayhi, has enumerated the following as major sins:

It is noteworthy to remember that some scholars have also said that there is, in fact, no such thing as a minor sin and that all sins are major. Another important thing to keep in mind is that no major sin remains a sin after sincere tawbah and that consistency in any sin will result in a major sin.[2]
For a detailed list on the major sins, it is recommended that you read a book called “Al-Kabaa’ir” by Imam Dhahabi. This book can be read from the following link: http://www.islamguiden.com/arkiv/majorsins.pdf    
 
And Allah Ta’āla Knows Best
Muhammad Haris Siddiqui
Student Darul Iftaa
Melbourne, Australia
Checked and Approved by,
Mufti Ebrahim Desai.