major surgery vs surgery

major surgery and surgery both are nouns.

Word	Noun	Adjective	Verb	Adverb
major surgery	Yes	No	No	No
surgery	Yes	No	No	No

As nouns, surgery is a hypernym of major surgery; that is, surgery is a word with a broader meaning than major surgery:

major surgery: any surgical procedure that involves anesthesia or respiratory assistance
surgery: a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body

Other hypernyms of major surgery include operation, surgical operation, surgical procedure, surgical process.

major surgery (noun)	surgery (noun)
any surgical procedure that involves anesthesia or respiratory assistance	a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body
	a room in a hospital equipped for the performance of surgical operations
	a room where a doctor or dentist can be consulted
	the branch of medical science that treats disease or injury by operative procedures