Sacrifice:An action that in objective terms costs you more than you actually benefited.
Sacrifice is sometimes understood as an action that costs you a lot. The problem with this kind of definition is that the only objective part of the definition is that there is a cost involved, and all actions involve some kind of cost. The qualifier "a lot" is a highly subjective judgment. If the value gained is significantly higher, you may not think of it as "a lot". $50 may be a lot for a dinner, but nothing compared to a new house. The cost is actually measured relative to the benefit itself. And then the only objective distinction to be made is whether it was greater or less than the benefit gained.
Sacrifice is often used to describe altruistic actions. One performs an altruistic act for the sake of the morality, and not because you benefit yourself. If you did benefit, your actions would be viewed as self-motivated, instead of others-motivated. So altruism is a sacrificial ethics, demanding that your moral actions cost you more than you benefit in objective terms. Of course, it might be said that you benefit "morally" by the actions, and so it isn't truly a sacrifice. But this is not measured by a standard of how it improves your life.
Under an altruistic ethics, it is desirable to suggest all of your others-oriented actions are not self-motivated. So people have an incentive to suggest they are sacrificing even when they aren't. Parent's will sometimes talk about the "sacrifices" they make for their children. While in some cases there may be actual sacrifices, often they are just describing the fact that they are incurring "large" costs for the children. The question of whether a sacrifice is really being made is more complicated, and depends partially on the benefits gained by the actions.