Wiktionary
adv. Of one’s own free will, of one’s own volition. n. A document issued by the Pope on his own initiative directed to the Roman Catholic Church.
Wikipedia
A motu proprio ( Latin for: "on his own impulse") is a document issued by the Pope (or by a monarch) on his own initiative and personally signed by him.
When issued by the Pope, a motu proprio may be addressed to the whole Church, to part of it, or to some individuals. A document issued motu proprio has its legal effect even if the reasons given for its issuance are found to be false or fraudulent, a fact which would normally render the document invalid. Its validity is based on its issuance by the pope by his own initiative, not upon the reasons alleged.
The first motu proprio was promulgated by Pope Innocent VIII in 1484. It continues to be a common form of Papal rescripts, especially when establishing institutions, making minor changes to law or procedure, and when granting favours to persons or institutions.
Usage examples of "motu proprio".
I have reference to matters outlined in the Motu proprio of Pope Celestine the Eighth, of happy memory, given on the Feast of the Divine Overshadowing of the Holy Virgin, Anno Domini 3735, and beginning with the words he paused to look through the papers on his desk Ab hac planeta nativitatis aliquos filios Ecclesiae usque ad planetas solium alienorum iam abisse et numquam redituros esse intelligimus.