A proof is just evidence something is true.
In math, evidence is having showed the steps involved to logically take the assumptions and reach the conclusion. Math tends to be more absolutist. If you can't prove beyond a shadow of a doubt your theorem is true (or that it is false), you must not speak of it at all.
In science, evidence is an experiment or a strong case for it to be true. It's closer to what you might find in a court of law. Generally, you can get away with making very strong assumptions without proof, so long as there is no direct evidence against you. In a scientist's eyes, the Riemann Hypothesis is true.
To derive is just another way to say you get a conclusion from taking logical steps. It's pretty much a synonym. Just as a theorem is a lemma is a corollary.