Altruism is the principle and moral practice of concern for the welfare and/or happiness of other human beings or animals, resulting in a quality of life both material and spiritual. It is a traditional virtue in many cultures and a core aspect of various religious and secular worldviews. However, the object(s) of concern vary among cultures and religions. In an extreme case, altruism may become a synonym of selflessness, which is the opposite of selfishness.

The word "altruism" was popularized (and possibly coined) by the French philosopher Auguste Comte in French, as altruisme, for an antonym of egoism. He derived it from the Italian altrui, which in turn was derived from Latin alteri, meaning "other people" or "somebody else".

Altruism in biological observations of populations of organisms is an individual performing an action which is at a cost to themselves (e.g., pleasure and quality of life, time, probability of survival or reproduction), but benefits, either directly or indirectly, another individual, without the expectation of reciprocity or compensation for that action.

Altruism can be distinguished from feelings of loyalty or concern for the common good. The latter are predicated upon social relationships, whilst altruism does not consider relationships. Much debate exists as to whether "true" altruism is possible in human psychology. The theory of psychological egoism suggests that no act of sharing, helping or sacrificing can be described as truly altruistic, as the actor may receive an intrinsic reward in the form of personal gratification. The validity of this argument depends on whether intrinsic rewards qualify as "benefits".

The term altruism may also refer to an ethical doctrine that claims that individuals are morally obliged to benefit others. Used in this sense, it is usually contrasted with egoism, which claims individuals are morally obligated to serve themselves first. Effective altruism is the use of evidence and reason to determine the most effective ways to benefit others.