Superstratum

In language contact studies, a superstratum (language) is a substratum language that was spoken by a politically dominant small group of speakers. Thus, while French has a Gaulish substratum (i.e. Gaulish influenced French but its speakers were not dominant), it has a Frankish superstratum (because the Frankish speakers dominated France for centuries until they gave up their language and adopted French).

Comment
Superstratum is usually opposed to substratum, as two different kinds of strata, but whether the speakers of the shifting language were politically dominant, equal or subordinate is not directly related to the linguistic outcome.

Origin
The term was coined by Walther von Wartburg in 1933.

Other languages
German Superstrat