What is Sacrifice?

Many times this thing has come in my mind, I tried talking with many people but yet I haven't found the answer which can satisfy me fully so I'm trying the same thing here too.

What is sacrifice? Is it selfless? or is it selfish? What are the terms and conditions of sacrifice? Does it definition change under circumstances?

I found this while browsing and to me it tells few things about sacrifice...

Sacrifice has many definitions. 
A mother sacrifices for her children because her love is so immense that words cannot describe.
A man/woman sacrifices for his/her spouse because he/she cares to keep the relationship going.
A sibling cares for the fellow sibling so he/she can sacrifice.
A leader sacrifices for his country.
A friend sacrifices for anothet
A lover sacrifices for his/her other half. 

Sacrifice can come in any form. Obviously the definition differs from one relationship to other. Sacrifice is a selfless act to keep something they love going good. Sounds selfish in a way. But it is for the betterment for the bond to keep going. One might go through pain and hurt through process of sacrifice but it is worth it.
But the deal is in today's society people mix up the terms generosity and sacrifice. Generosity is a selfless act based on a charitable mind. No lose on either party. It is true that one party gains while the other doesn't. But the one who goves doesnt under go any pain instead gains a happiness. 
But in the case of sacrifice it is a struggle of "should i? Or shouldn't i?". Sacrifice is a case where if one party doesnt the the other party must do it if not it might lead to some sort of mess.
Generosity is a free thought of "i will give". 
Generosity and sacrifice are same actions, different procedures, same result, different feelings on the party who gives.
Too complicated to explain. Guess i made a certain point.