(From Oxford dictionary):
1.an act of slaughtering an animal or person or surrendering a possession as an offering to a deity:they offer sacrifices to the spirits [mass noun]:the ancient laws of animal sacrifice an animal, person, or object offered in the act of sacrifice: a flat cake offered by the Romans as a sacrifice to their gods
2. Christian Church: Christ’s offering of himself in the Crucifixion.
the Eucharist regarded either (in Catholic terms) as a propitiatory offering of the body and blood of Christ or (in Protestant terms) as an act of thanksgiving.
3. an act of giving up something valued for the sake of something else regarded as more important or worthy: (2)
The term is also used metaphorically to describe selfless good deeds for others or a short term loss in return for a greater power gain, such as in a game of chess. Recently it has also come into use as meaning 'doing without something' or 'giving something up' (2)