Some definitions specify a self-sacrificial nature to altruism and a lack of external rewards for altruistic behaviors.
sˌʌm dˌɛfɪnˈɪʃənz spˈɛsᵻfˌaɪ ɐ sˈɛlfsˌækɹɪfˈɪʃəl nˈeɪtʃɚ tʊ ˈɔːltɹuːˌɪzəm ænd ɐ lˈæk ʌv ɛkstˈɜːnəl ɹᵻwˈɔːɹdz fɔːɹ ˌɔːltɹuːˈɪstɪk bᵻhˈeɪvjɚz.