Begging the question is a logical fallacy that occurs when an argument relies on an assumption that is itself unproven, thereby creating a circular argument. It assumes the conclusion as a premise without providing any evidence to support it.
Consider the following example: “Evolution is true because science says so, and science is true because it is based on evolution.” This argument is circular because it relies on the truth of evolution to support the truth of science, and vice versa. It fails to provide any independent evidence to support either claim.