Someone who is altruistic always puts others first. An altruistic firefighter risks his life to save another's life, while an altruistic mom gives up the last bite of pie so her kid will be happy.
This word comes from the Old French altruistic and means "other people" and before that the Latin alter, which means "other." Our current word comes from the nineteenth century and comes from philosophy. It means that it's important for people to be concerned with the good of others and to act unselfishly. When the word refers to the animal world, it means behavior that may actually harm the individual animal but will benefit the species in general. This is a truly selfless act!