A scientific law is a description of an observed phenomenon. A scientific theory is an explanation of why an observed phenomenon occurs.

While a “law” sounds more authoritative than a “theory” there is no greater certainty in one over the other. This perception arises out of the other uses we have for these two words.

Let’s look at some selected definitions of the word “theory”. These are clearly in conflict with each other. A “scientific acceptable principle” is very different from “an unproven assumption”. For the record, the “theory of evolution” is the former.

theory \ ˈthē-ə-rē \

1: a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena

3a: hypothesis assumed for the sake of argument or investigation

3b: an unproved assumption

Theory | Definition of Theory by Merriam-Webster

So, what about a scientific law?

law \ ˈlȯ \

6a: a statement of an order or relation of phenomena that so far as is known is invariable under the given conditions

Law | Definition of Law by Merriam-Webster

The phrase “so far as is known” is more or less consistent with the level of certainty that you find in “plausible or scientifically acceptable general principle”.

Enjoy the following video from Ted-Ed on the subject.