Substratum

The substratum of a language is the language that influenced it by interference through shift at an earlier point. For example, French has a Gaulish substratum, i.e. the speakers of the Gaulish language that learned Latin/French influenced the language they learned by involuntarily carrying over some (especially phonological and syntactic) structural properties to it.

Term properties
An alternative form of the term is substrate. The relational adjective is substratal.

Origin
The term was coined by Graziadio I. Ascoli in 1867.

Other languages
German Substrat