Fact-Value Distinction: In philosophy, the ontological distinction between what is (facts) and what ought to be (values). David Hume gave the distinction its classical formulation in his dictum that it is impossible to derive an “ought” from an “is.” See also naturalistic fallacy.1
Better understood as "what is" (fact) and "what ought to be" (value), the fact/value distinction is the thin line between what is truth and what is right. It is the source of conflict between science and ethics. In its most basic sense, fact can be defined as the inarguable truths of our physical world - the material surroundings which one detects via the senses. By examining our reality through scientific methods, we hope to empirically and logically verify truths and thus to compile a collection of "knowledge". Value, on the other hand, is not accessible via the senses; it can only be derived through one's own subjective reasoning about ethics. Unlike fact, value cannot be proven true or false by any sort of scientific method. Rather, it must be compared against one's own faith or ethical worldview in order to draw personal conclusive results.2
1. Encyclopedia Britannica; 2. rit.edu; For more, see wikipedia here.