Altruism:A system of ethics that promotes actions that benefit other people, usually requiring self-sacrifice.
In practice, the actions cannot benefit you, or they will be suspected of being performed for "selfish" reason. The actions must be a sacrifice, requiring a larger cost than a benefit to you. This shows that you are not doing it for personal gain, and are doing it because you believe it is the right thing to do.
Altruism is often defended from the viewpoint that there is a necessary conflict of interests between people. That means that one man's benefit must be at some else's expense. Under that view, you have to choose between sacrificing others to yourself, or sacrificing yourself to others. Altruism is the latter choice, which allegedly removes conflicts by having people voluntarily choose to sacrifice.
Altruism is sometimes confused with benevolence, making it appear that altruism is about being friendly and kind to others. But benevolence is completely compatible with a self-interested ethics, since we benefit from positive interactions with other people, whether romantic, educational, economic, or many others. This confusion works to create the impression that you must choose between self-sacrifice for other people, and not being friendly, caring, or compassionate.