Old High German (OHG, German: Althochdeutsch, German abbr. Ahd.) is the earliest stage of the German language, conventionally covering the period from around 500 to 1050 CE. Coherent written texts do not appear until the second half of the 8th century, and some treat the period before 750 as "prehistoric" and date the start of Old High German proper to 750 for this reason. There are, however, a number of Elder Futhark inscriptions dating to the 6th century (notably the Pforzen buckle), as well as single words and many names found in Latin texts predating the 8th century.

The main difference between Old High German and the West Germanic dialects from which it developed is that it underwent the Second Sound Shift or High German consonant shift. This is generally dated very approximately to the late 5th and early 6th centuries—hence dating its start to around 500. The result of this sound change is that the consonantal system of German remains different from all other West Germanic languages, including English and Low German. Grammatically, however, Old High German remained very similar to Old English, Old Dutch, and Old Saxon.

The High German languages (in German, Hochdeutsche Sprachen) or the High German dialects (Hochdeutsche Mundarten/Dialekte) are any of the varieties of standard German, Luxembourgish and Yiddish, as well as the local German dialects spoken in central and southern Germany, Austria, Liechtenstein, Switzerland, Luxembourg and in neighboring portions of Belgium and the Netherlands (Ripuarian dialects in southeast Limburg), France (Alsace and northern Lorraine), Italy, Denmark, and Poland. The language is also spoken in diaspora in Romania (Transylvania), Russia, the United States, Brazil, Argentina, Chile, and Namibia.

As a technical term, the "high" in High German is a geographical reference to the group of dialects that forms "High German" (in the broader sense), out of which developed standard High German (in the narrower sense), Yiddish and Luxembourgish. It refers to the upland and mountainous 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 on the lowlands and along the flat sea coasts of the north. 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 is now a standardized language), and High Franconian which is a transitional dialect between the two.