In popular speech, a willingness to sacrifice one’s own interests for those of others. It is this sense that is relevant to discussions of, for example, the evolutionary origins and role of altruism in animals.
Philosophically, altruism is rather a view about what one ought to do, and contrasts with egoism and universalism.
It is a form of consequentialism and prescribes that one should act so as to maximize the happiness or welfare of people (or possibly living creatures) in general, oneself alone excluded. Only these last three words distinguish altruism from universalism or utilitarianism, and in principle it has varieties corresponding to the varieties of these, and is open to many of the same objections.
It also faces objections which mirror some of those facing egoism; for example, to what extent it is really practicable.