pe·ti·ti·o prin·ci·pi·i (pə-tĭsh'ē-ō' prĭn-sĭp'ē-ē', -ē-ī') n.
Logic The fallacy of assuming in the premise of an argument that which one wishes to prove in the conclusion; a begging of the question.
[Medieval Latin petītiō prīncipiī : Latin petītiō, request + Latin prīncipiī, genitive of prīncipium, beginning.]