Possibly what ever you want it to be?
I recall walking into a demo at a Bristol show, where as I turned to walk out because of the (to me) excessive one note bass, screechy treble, I had to skirt round two people, one saying “Wow, great PRaT.”
As IB says, there is no definition. The closest I have found is here PRaT: Audio System Timing In Context | The Ear if @Richard.Dane allows the link.