What a hard question xD Love can't be defined because its sooooo broad, but I think that's the point. There's no telling what love can make us feel or do, and so we can't describe what love is, because it's different for everybody.
My definition of love (romantic kind) would be when you look into the person's eyes and you can see your future with them. You feel like you'll marry them one day and that you'll trust that they'll promise to stay with you through all the good and bad times, for the rest of your life. Exactly like wedding vows :) Love is when you can't imagine life without them, because they are so involved in your life and you are in theirs. It's when reality is better than your dreams. It's when their feelings can affect yours - you are happy when they are, sad when they seem down. You care about them. You dedicate your life to making them happy. It's when they can be two people at once - a lover and a best friend. And most importantly.. it's about sacrifice and commitment.