In a single sentence...
Love is wisely acting in the best interest of others.
Love is the commitment to the good of someone.
"Someone" because that someone could be oneself.
Whether it is a simply act of kindness or the forfeiture of your physical life, it is the fact that the action was taken in the best interest of another person that makes it an act of love. However, I say "wisely" because such a statement could be taken to an extreme that is anything but wise. For one thing, just because you feel like something is in someone else's best interest doesn't mean that it actually is. Giving a beggar on the street corner money, even though it feels like a kindness, might actually be the worst possible thing you could do. Also, acting in someone else's best interest when it means disaster for yourself isn't usually a wise thing to do. That's why Jesus said to love others as you love yourself. Notice in that directive that loving others is predicated on loving yourself and therefore, acting in someone else's best interest is not necessarily in conflict with your own interests. Indeed, most often, there is no real conflict of interest between righteous men.