What is the definition of a law in science?

The definition of a law in science is a tricky one. A scientific law however does has very defined criteria. One definition of a law taken from Lincoln et al., 1990 states that a scientific law is: 

An empirical generalization; a statement of biological principle that appears to be without exception at the time it is made, and has become consolidated by repeated successful testing; rule.

Based on this definition, a law is any scientific statement or formula that has held true after undergoing repeated experiment and testing over a long period of time. It should be noted however that later findings may counter a law and prove that it only holds true under a certain circumstance or set of circumstances. A good example of this is Newton's law of gravity which holds true here on earth, but does not hold true in the larger context of gravity in space and in the universe. Hope this helps!