"The big overarching linguistic theory to which Grimm looked when trying to determine the common ground between the Germanic and the Romance languages, is that there was an original language from which the ancient and modern languages developed. This language no longer exists and all attempts to reconstruct it are strictly theoretical. To show that it is not a real language, it is called "Proto"; in this case, ProtoIndoEuropean, or [PIE] In the 18th century, there were already people who had come up with the beginnings of the concept, thinking Sanskrit was the original language, but it was left to classically educated Sir William Jones, who was stationed in India as Chief Justice, to formulate the basic idea" -- http://web.archive.org/web/*/http://classics.allinfoabout.com/articles/pie.html
Proto Indo European