Chemistry majors, please stand up

If I somehow slipped back in time, and met Mrs Tan, my Science teacher, I'd ask her, "Mrs Tan. What is chemistry?"

Recently this term has been bandied about quite a bit.

What is chemistry? How does one define it? Is it just being able to click? Being able to talk? Feeling like you totally have known each other for much longer than the actual fact? Is it sexual?

What does it mean when a girl tells a guy that there is no chemistry left?

Is it the end?