There are multiple definitions - click on the translation that matches your context.
petitioner means:
substantivo
partial
noun
The term "petitioner" is broader in meaning than "impetrante". In Brazil, "impetrante" is the person who files a petition for a writ of mandamus ("mandado de segurança"), a writ of habeas corpus or a writ of habeas data.
someone who makes a formal written request to a court or other official body (=a petition)
The hearing in the proceedings in which John F. Baudouine appears as a petitioner in bankruptcy was resumed yesterday morning.