In the Science of ethology (the study of animal behaviour), and more generally in the study of social evolution, altruism refers to behavior by an individual that increases the fitness of another individual while decreasing the fitness of the actor.
ɪnðə sˈaɪəns ʌv iːθˈɑːlədʒi ðə stˈʌdi ʌv ˈænɪməl bᵻhˈeɪvjɚ, ænd mˈoːɹ dʒˈɛnɚɹəli ɪnðə stˈʌdi ʌv sˈoʊʃəl ˌɛvəlˈuːʃən, ˈɔːltɹuːˌɪzəm ɹᵻfˈɜːz tə bᵻhˈeɪvjɚ baɪ ɐn ˌɪndᵻvˈɪdʒuːəl ðæt ɪŋkɹˈiːsᵻz ðə fˈɪtnəs ʌv ɐnˈʌðɚɹ ˌɪndᵻvˈɪdʒuːəl wˌaɪl dˈiːkɹiːsɪŋ ðə fˈɪtnəs ʌvðɪ ˈæktɚ.