Why is Einstein's description of gravity called a theory, while Newton's is a law?

Albert Einstein came up with the theory of general relativity to explain the law of gravity, whilst Newton's three laws of gravity is universal. To understand this further, it's best to understand it in scientific terms.

The weird thing about science is that words that are used in a colloquial sense may have a completely opposite definition in scientific terms.

A law in science is a constant and invariable statement that is universal. Wherever you may be in the universe, Newton's three laws of gravity will always be applied.

The word "theory" doesn't imply conjecture or an idea someone made up after a night of drinking. In science, a theory is the highest level of certainty behind mathematical proof - - which isn't even a part of science, obviously. A theory has to be substantiated by all available evidence and contradicted by none. All theories also have to have to be falsifiable. For this reason, theories can never be proven. Einstein's theory of general relativity has great predictive power, but in some cases, the predictions aren't always constant. Theories are often revised to fit new available evidence.

Albert Einstein came up with the theory of general relativity to explain the law of gravity, whilst Newton's three laws of gravity is universal.