What is Petitio?

Legal Definition
Lat. In the civil law. The plaintiff's statement of his cause of action in an action in rem. Calvin. In old English law. Petition or demand; the count in a real action; the form of words in which a title to land was stated by the demandant, and which commenced with the word "veto." 1 Reeve, Eng. Law, 176.
-- Black's Law Dictionary
Legal Definition
A petition; a demand.
-- Ballentine's Law Dictionary