A grey area is a term for a border in-between two or more things that is unclearly defined, a border that is hard to define or even impossible to define, or a definition where the distinction border tends to move. There are several flavors of grey areas:

• A grey area of definitions signifies a problem of sorting reality into clearly cut categories. Example: where is the border between erotica and pornography?
• A grey area of law is an area where no clear legislation or precedent exists, or where the law has not been applied in a long time thus making it unclear if it is applicable at all. See non liquet and case of first impression.
• A grey area of ethics signifies an ethical dilemma, where the border between right and wrong is blurred. Example: is killing always abominable?