exact science definitions

Merriam Webster's

noun Date: 1843 a science (as physics, chemistry, or astronomy) whose laws are capable of accurate quantitative expression

Collin's Cobuild Dictionary

If you say that a particular activity is not an exact science, you mean that there are no set rules to follow or it does not produce very accurate results. Forecasting floods is not an exact science. N-SING: usu with brd-neg