difference between operation and surgery

operation - (мэс засал )
[countable] the process of cutting open a part of a person's body in order to remove or repair a damaged part
      ----- Will I need to have an operation?
[countable] an organized activity that involves several people doing different things
     -----a security operation
surgery - (мэс ажилбар, хагалгаа )
medical treatment of injuries or diseases that involves cutting open a person's body and often removing or replacing some parts; the branch of medicine connected with this treatment
     ----- She required surgery on her right knee.\

difference between operation and surgery

• i don't think there is a difference.
• no difference...

• In AE, "surgery" refers to the overall process or the field of medicine. It is usually uncountable. "Operation" refers to one specific surgical procedure. It is always countable.
  
  So, both are correct. The students could watch the surgery (all the activity that took place during their time in the operating room) or the operation (one specific procedure). The choice, as is so often the case, depends on what you want to say.
  
  There are contexts in which either one can be used. Someone can say "I had surgery this morning" (note that there is no article, since "surgery" is uncountable) or "I had an operation this morning." In AE, one is unlikely to say "I had several surgeries this morning," and would never say "I had a surgery this morning."
  
  The first version, "I had surgery ...," allows for the possibility that this person had more than one operation, but that would be unusual. So, in this case, they mean pretty much the same thing.
  
  BE usage may be different, aside from the fact that in BE "surgery" can also mean what we Yanks call a doctor's office.

• The term operation has several meanings: a general action, business operation, flight operation and so on. Surgery is the branch of medicine concerned with diagnosis and treatment of injuries, pathological conditions by means operative procedures. Surgery operation is on of these procedures, it may be simply called "surgery".

• an operation can mean many things like .. a job that needs to be done and so on .. but surgery is when the cut you open to fix a problem. 

• Operation is a synonym for surgery, meaning exactly the same thing in regards to surgery. An operation, or a surgical procedure, is when a trained physician opens the body to provide therapy or to repair a problem within the body.