So, just what exactly is love?

Inspired by this question.

What is love (and for this context, we’re talking more in the realm of romantic love, despite my issues with that term – but not parental love or sibling love)? Is it infatuation that grows into a stronger bond? Is infatuation definitely, unequivocally, not love? What exactly is the difference between a romantic relationship based on true love and best friends that are also erotically into each other? Psychologically, what is it – is it just a combination of chemicals in the brain?

Now, before you say “If you have to ask, this isn’t it” or “You’ll know it when you feel it”, let me point out that our current understanding of romantic love hasn’t always been around. Those statements assume so much that the person you’re saying it to is on the same page as you are, culturally, linguistically, philosophically. So don’t assume I’ll “just know” – explain it to me (since that’s the whole point of the thread).