A logical fallacy in which the definition of a term is self-contradictory. This is a good way to tell if someone has no idea what he's talking about.

Example: "Daytime is when you can see the sun and no stars." The sun is a star.

To prove that the definition is self-contradictory, proceed in a manner similar to that for Inconsistency: assume one condition of definition is true, and show that another must be false.

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