The present-day distribution of continental High German languages:
Central German dialects
Upper German dialects
As a technical term, the "high" in High German is a geographical reference to the group of dialects that forms "High German" (i.e. "Highland" German), out of which developed Standard German, Yiddish and Luxembourgish. It refers to the Central Uplands (Mittelgebirge) and Alpine areas of central and southern Germany; it also includes Luxembourg, Austria, Liechtenstein and most of Switzerland. This is opposed to Low German, which is spoken in the lowlands and along the flat sea coasts of the North German Plain.
High German in this broader sense can be subdivided into Upper German (Oberdeutsch, this includes the Austrian and Swiss German dialects), Central German (Mitteldeutsch, this includes Luxembourgish, which itself is now a standard language), and High Franconian German, which is a transitional dialect between the two.
High German (in the broader sense) is distinguished from other West Germanic varieties in that it took part in the High German consonant shift (c. AD 500). To see this, compare English/Low German (Low Saxon) pan/Pann with Standard German Pfanne ([p] to [p͡f]), English/Low German two/twee with Standard German zwei ([t] to [t͡s]), English/Low German make/maken with Standard German machen ([k] to [x]). In the southernmost High Alemannic dialects, there is a further shift; Sack (like English/Low German "sack/Sack") is pronounced [z̥ak͡x] ([k] to [k͡x]).