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In philosophy, meta-ethics (sometimes called "analytic ethics") is the branch of ethics that seeks to understand the nature of ethical properties, and ethical statements, attitudes, and judgments. Meta-ethics is one of the three branches of ethics generally recognized by philosophers, the others being ethical theory and applied ethics. Ethical theory and applied ethics make up normative ethics. Meta-ethics has received considerable attention from academic philosophers in the last few decades.
While normative ethics addresses such questions as "Which things are (morally) good and bad?" and "What should one do?", thus endorsing some ethical evaluations and rejecting others, meta-ethics addresses the question "What is (moral) goodness?", seeking to understand the nature of ethical properties and evaluations.

Moral realism
Moral anti-realism holds that there are no objective moral values. This view comes in just three possible variants:
Subjectivism, non-cognitivism, and error theory are the only forms of anti-realism: If there are no objective values, this must be either because ethical statements are subjective claims (as subjectivists maintain), because they are not genuine claims at all (as non-cognitivists maintain), or because they are mistaken objective claims. The only alternative is for ethical statements to be correct objective claims, which entails moral realism.
Another way of categorizing meta-ethical theories distinguishes between monistic theories (in which there is one true, or at least one highest, good) and pluralistic theories.
Value pluralism contends that there are two or more genuine values, knowable as such, yet incommensurable, so that any prioritization of these values is either non-cognitive or subjective. A value pluralist might, for example, contend that both the life of a nun and that of a mother realize genuine values (in an objective and cognitivist sense), yet there is no purely rational measure of which is preferable. A notable proponent of this view is Isaiah Berlin.

Ethical subjectivism, which holds that moral statements are made true or false by the attitudes and/or conventions of observers. There are several different versions of subjectivism, including:

• Moral relativism (compare "cultural relativism"): This is the view that for a thing to be morally right is for it to be approved of by society; this leads to the conclusion that different things are right for people in different societies and different periods in history. Though long out of favor among academic philosophers, this view has been popular among anthropologists, such as Ruth Benedict.
  Divine command theory: Another subjectivist theory holds that for a thing to be right is for a unique being, God, to approve of it, and that what is right for non-God beings is obedience to the divine will. This view was criticized by Plato in the Euthyphro (see the Euthyphro problem) but retains some modern defenders (Robert Adams, Philip Quinn, and others).
  Individualist subjectivism: Another view is that there are as many distinct scales of good and evil as there are subjects in the world. This view was put forward by Protagoras.
  Ideal observer theory: Finally, some hold that what is right is determined by the attitudes that a hypothetical ideal observer would have. An ideal observer is usually characterized as a being who is perfectly rational, imaginative, and informed, among other things.
  Non-cognitivism, which holds that ethical sentences are neither true nor false because they do not express genuine propositions. Non-cognitivism encompasses:
  
  
  
  • Emotivism, defended by A.J. Ayer and C.L. Stevenson, which holds that ethical sentences serve merely to express emotions. So "Killing is wrong" means something like, "Boo on killing!"
    Prescriptivism, defended by R.M. Hare, which holds that moral statements function like imperatives. So "Killing is wrong" means something like, "Don't kill!"
    Quasi-realism, defended by Simon Blackburn, which holds that ethical statements behave linguistically like factual claims and can be appropriately called "true" or "false", even though there are no ethical facts for them to correspond to. Projectivism and moral fictionalism are related theories.
    Error theory (also known as moral nihilism) holds that ethical claims are generally false. They hold that there are no objective values---that nothing is morally good, bad, wrong, right, etc.---because there are no moral truths. For example, an error theorist would say that murder is not wrong, but it's not right either. J. L. Mackie is probably the best-known proponent of this view. (Note that error theory is also sometimes associated with moral skepticism, but they have some slight differences. Error theories are a type of moral skepticism, but there are other types of moral skepticism.)