Wiktionary
hard science
n. (context informal English) The natural science and physical sciences that use the scientific method and experiments to test theory. Examples include mathematics, biology, physics, chemistry and geology.
Usage examples of "hard science".
Every time I hear the term ``hard science fiction,'' I think to myself, ``Hard?
That novel has been praised for its unusually deft blend of complex hard science concepts and even more complex characters.
One of the best writers of hard science fiction is Fred Pohl, and he never even finished high school.
I'd classify most of James Blish's work under the present heading, though of course he writes topflight hard science whenever he wants to.