Naturalistic fallacy

Etymology

Coined by philosopher G.E. Moore in “Principia Ethica” (1903), from Latin "natura." Moore argued that “good” is a simple, indefinable property, so any attempt to equate it with a natural feature commits a fallacy. The name is often loosely stretched to cover related errors.

noun phrase · /NACH-er-uh-LIS-tik FAL-uh-see/
The mistake of defining moral goodness in terms of some natural property, such as what is pleasant, desired, or evolutionarily useful.
What it names
A precise error in ethical reasoning — treating “good” as if it just meant a factual quality — that is easy to commit and hard to notice.
Example
Arguing that because a behavior is “natural” it must be morally acceptable commits the naturalistic fallacy.
Related idea
Frequently conflated with the appeal to nature, which claims that natural equals good, and with Hume’s is–ought problem; Moore’s point is narrower and definitional — that goodness cannot be reduced to any factual property at all.