Fallacy of treating the term "good" (or any equivalent term) as if it were the name of a natural property.
In 1903 G.E. Moore presented in Principia Ethica his "open-question argument" against what he called the naturalistic fallacy, with the aim of proving that "good" is the name of a simple, unanalyzable quality, incapable of being defined in terms of some natural quality of the world, whether it be "pleasurable" ( John Stuart Mill ) or "highly evolved" ( Herbert Spencer ). Since Moore's argument applied to any attempt to define good in terms of something else, including something supernatural such as "what God wills," the term "naturalistic fallacy" is not apt. The open-question argument turns any proposed definition of good into a question (e.g., "Good means pleasurable" becomes "Is everything pleasurable good?")
Moore's point being that the proposed definition cannot be correct, because if it were the question would be meaningless.