Published in 1903, the Principia Ethica is philosopher G. E. Moore's most famous book. It promotes non-naturalist moral realism, or simply non-naturalism. This is the view that goodness exists in some objective sense but cannot be discovered or defined by empirical science. In other words, moral truths are totally distinct and isolated from all other kinds of knowledge (scientific, religious, and even philosophical).
The title is a (fairly hubristic) allusion to Newton's Principia Mathematica.
It was a moral Bible of sorts for the Bloomsbury Group. Virginia Woolf called it "the book that made us all so wise and good."