Thus, both terms refer to a situation where an intrusive language establishes itself in the territory of another, typically as the result of migration. Whether the superstratum (the local language persists and the intrusive language disappears) or the substratum (the local language disappears and the intrusive language persists) case applies will normally only be evident after several generations , during which the intrusive language exists within a diaspora culture. In order for the intrusive language to persist (substratum case), the immigrant population will either need to take the position of a political elite or immigrate in significant numbers relative to the local population. (i.e. the intrusion qualifies as an invasion or colonisation, an example would be the Roman Empire giving rise to Romance languages outside of Italy, displacing Gaulish)
The superstratum case refers to elite populations which eventually adopt the local language (an example would be the Burgundians and Franks in France, who eventually abandoned their Germanic dialects in favour of Romance).
In a typical case of substrate interference, a language A occupies a given territory, and another language B arrives in the same territory (brought, for example, with migrations of population). Then language B begins to supplant language A: the speakers of language A abandon their own language in favour of B, generally because they believe that it is in their best (e.g. economic, political, cultural, social) interests to do so. During the language shift, however, the receding language A still influences language B (for example, through the transfer of loanwords, place-names, or grammatical patterns from A to B).
For example, Gaulish is a substratum of French. A Celtic people, the Gauls, lived in the current French-speaking territory before the arrival of the Romans. Given the cultural, economic and political prestige which Latin enjoyed, the Gauls eventually abandoned their language in favour of Latin, which evolved in this region until eventually it took the form of Modern French. The Gaulish speech disappeared, but it remains detectable in some French words (approximately ninety) as well as place-names of Gaulish origin.
Linguistic substrata may be difficult to detect, especially if the substratum language and its nearest relatives are extinct. For example, the earliest form of the Germanic languages may have been influenced by a non-Indo-European language, purportedly the source of about one quarter of the most ancient Germanic word-stock; see Germanic substrate hypothesis.
Creole languages typically have multiple substrata, rarely homogeneous ones.
The term was coined by Walter von Wartburg.
It is also used to describe an imposed linguistic element, akin to what English underwent after 1066 with Norman. The Neo-Latin and Neo-Greek coinages adopted by European languages (and now, languages worldwide) to describe scientific topics (anatomy, medicine, botany, zoology, all the '-ology' words, etc.) can also be termed a superstratum, although for this last, adstratum would be a better choice. The term adstratum refers to a language which is equal in prestige to another. Generally the term is used only when speaking about languages in a particular country or geopolitical region. For example, early in England's history, English and Norse had an adstratal relationship.
The phenomenon is relatively rare today, since modern nations generally have only one dominant language (often corresponding to the dialect of the capital). In India, where dozens of languages are widespread, many could be said to share an adstratal relationship, although Hindi is certainly dominant in North India. A more accurate example would be the situation in Belgium, where the French and Dutch languages have roughly the same status, and could justifiably be called adstrates.