What is Sacrifice?

What is Sacrifice?
Once I asked someone wise

Is it the student, who goes for graduate study
Leaving his family and salary, while his future is still muddy

Is it the sportsman, who puts himself in a cage
Before he reaches the big stage

Is it the soldier, who is ready to die for his country
In turn making the supreme sacrifice, adorning the cemetery

Is it an emotion, which can be defined
Or an act, which can't be theorized

Elton John sang its not a simple word, understand you must
Otherwise how can you understand your parents, who always put you first?