petitio principii

Latin, from petitio "petition" (see petition (n.)) + genitive of principium (see principle (n.)). It translates Greek to en arkhē aiteisthai "an assumption at the outset."

In logic, the assumption of that which in the beginning was set forth to be proved; begging the question: a fallacy or fault of reasoning belonging to argumentations whose conclusions really follow from their premises, either necessarily or with the degree of probability pretended, the fault consisting in the assumption of a premise which no person holding the antagonistic views will admit. [Century Dictionary]