There are varying definitions of major surgery, and what constitutes the difference between major and minor surgery. As a general rule, a major surgery is any surgery in which the patient must be put under general anesthesia and given respiratory assistance because he or she cannot breathe independently. In addition, major surgery usually carries some degree of risk to the patient's life, or the potential for severe disability if something goes wrong during the surgery.

Several other features can be used to distinguish major surgery from minor surgery. For example, in a major surgical procedure, significant resection or changes to the anatomy are involved, as in situations where organs are removed, or joints are rebuilt with artificial parts. Any penetration of the body cavity is treated as major surgery, as are extensive orthopedic surgeries on the extremities. Neurosurgery is generally considered major because of the risks to the patient, even though patients are not necessarily under general anesthesia during procedures on the brain.

Some examples of major surgery include: knee replacement, cardiovascular surgery, and organ transplants. These procedures carry definite risks from the patient such as infection at the site of the surgery, hemorrhage, or complications from the general anesthetic used. To reduce the potential for complications, major surgery takes place in a sterile operating room where very precise procedures are observed to reduce the risk of contamination and the patient is monitored by an anesthesiologist and a medical team for any signs of distress.

Recovery from major surgery can take several days or weeks. In the first few days, the patient is often hospitalized so that he or she can be monitored, and physical therapy may be offered to get the patient active, which reduces the risk of post surgical complications. After the patient is released, he or she may have to continue taking certain medications, and observing precautions during the healing period.

Preparation for major surgery can also be extensive. The patient usually needs to consult with a surgeon to talk about the procedure, the recovery time, and important aftercare instructions, and he or she will need to undergo a series of tests to confirm fitness for surgery. In emergency surgery where such lengthy preparations may not be an option, surgeons have to use their best judgment to make appropriate decisions for patients who are in critical need of surgery. A patient with abdominal bleeding, for example, may not be able to sign consent forms or to review documentation related to the surgery which would be needed to stop the bleeding.