In linguistics, a stratum or strate (Latin: layer) is a language that influences, or is influenced by another through contact. A substratum or substrate is a language which has lower power or prestige than another, while a superstratum or superstrate is the language that has higher power or prestige. Both substratum and superstratum languages influence each other, but in different ways. An adstratum or adstrate refers to a language that is in contact with another language in a neighbor population without having identifiably higher or lower prestige. The terms '‘superstrate'’ and ‘'adstrate'’ were first used by two different authors in 1932.

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 case (the local language persists and the intrusive language disappears) or the substratum one (the local language disappears and the intrusive language persists) 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 and many other languages).