Altruistic behaviour shows that you care about other people and will help them, even though this brings no advantage for yourself. We use it for acts and for people: an altruistic gift, an altruistic doctor. It is a formal adjective. Though actually, everyday speech sometimes stretches it to mean only “generous” or “nice,” which drops the strict “no gain for you” part. In careful use, that part stays. The matching noun for the idea is altruism; the person is an altruist.