Cornell realism is a view in metaethics, associated with the work of Richard Boyd, Nicholas Sturgeon, David Brink, and Peter Railton. There is no recognized and official statement of Cornell realism (Brink's Moral Realism and the Foundation of Ethics comes close), but several theses are associated with the view. In philosophy, ethics is commonly divided into two branches, normative ethics and meta-ethics. ...

1. Moral realism: Moral judgments are descriptive and cognitive judgments regarding suitably mind-independent and therefore objective moral facts, facts that do exist. This combines a view about moral judgments (they are belief-like mental states in the business of describing the world), a view about the existence of moral facts (they do in fact exist), and a view about the nature of moral facts (they are objective: independent of our cognizing them, or our stance towards them, etc.). This contrasts with expressivist or fictionalist theories of moral judgment (e.g., Stevenson, Hare, Blackburn, Gibbard, Kalderon), irrealist denials of the existence of moral facts (e.g., Mackie, Joyce, and the expressivists), and constructivist or relativist theories of the nature of moral facts (e.g., Firth, Rawls, Korsgaard, Harman).
2. Motivational externalism: Moral judgments needn't have any motivational force at all. A common way of explaining the thesis invokes the claim that amoralists are possible – that there could be someone who makes moral judgments without feeling the slightest corresponding motivation. This gives Cornell realists a simple response to Humean arguments against cognitivism: if moral judgments don't have motivational force in the first place, there is no reason to think they are non-cognitive states. Some, like Brink, add to this motivational externalism an externalism about normative reasons, holding that you can be under a moral requirement without having any normative reason to comply.
3. Naturalistic reductionism about metaphysics: Moral facts are (nothing over and above) natural facts. They fall within the province of the natural and social sciences. They are not supernatural (as in divine command theory) and they are not non-natural (as in Moore's Principia Ethica or Mackie's picture of a realist world).
4. Kripke-Putnam non-reductionism about semantics: There is no reductive connection between moral terms and concepts and natural terms and concepts. This gives Cornell realists a simple response to the charge that you cannot have naturalism without naturalistic fallacy: namely, that metaphysical reduction doesn't imply semantic reduction. This usually goes with a Kripke-Putnam semantic story: moral terms and concepts pick out certain natural properties in virtue of those properties standing in an appropriate causal (social-historical) relation to our tokenings of the terms and concepts.