[Woahhs]

Then why was the term "law" appropriated to name certain concepts? What distinction does it signify?

[Dimensio]

"Law" signifies a theory that has both withstood rigourous testing and is used as the basis for future hypothesis and theory. It does not mean "proven".

"Law" in science only means that falsifying it would create a bigger mess than would falsifying a theory that is not "law".

[BMCDA]

Theories, facts and laws in science are all different concepts and one doesn't change into the other.

Scientific facts are what have been carefully observed to be the case. This, however, does not mean that they cannot change if the observation methods advance.
Scientific laws are (often mathematical) descriptions of these facts and they are only descriptive and not prescriptive as the laws we are used to (like those that prohibit theft).
Finally, scientific theories are explanations of scientific facts.
So in other words, laws say what happens, while theories explain why it happens.