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Definition of altruism. 1  1 : unselfish regard for or devotion to the welfare of others charitable acts motivated purely by altruism. 2  2 : behavior by an animal that is not beneficial to or may be harmful to itself but that benefits others of its species.
Altruistic behaviour is therefore the opposite of egoistic behaviour. It involves intention to help others when used of human behaviour, but the fact that some animal behaviour is judged to be altruistic indicates that there are two possible bases to a definition: intentionality, and behavioural effects.