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.
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.
Also called (William Donald) Hamilton's genetical theory of social behavior, altruism is any behavior of an animal that may be disadvantageous for the individual, but that benefits others of its species.
The theory was espoused (as the 'law of mutual aid') by Prince Peter Kropotkin (1842-1921), a Russian philosopher; but the term was coined by Auguste Comte (1798-1857), French philosopher and social reformer.
A bird that warns the flock of impending danger by calling out, thereby making itself obvious to the predator, is nonetheless increasing the likelihood that its own kin (and their shared gene pool) will survive. Prince Peter Kropotkin believed that both animal and human survival depended on mutual aid, and recorded many examples from the animal kingdom.
Compare with: survival of the fittest
Also see: haplodiploid hypothesis, inclusive fitness, kin-selection theory