Material design is a design language developed by Google. A design language is an overarching scheme or style that guides the design of a complement of products or architectural settings.
Now is "material design" a proper noun? We are having that discussion at https://en.wikipedia.org/wiki/Talk:Material_design . I say yes, because "Material Design" as used in the Wikipedia article uniquely refers to Google's design language as defined on http://www.google.com/design/spec/material-design/introduction.html . While that specification can change, it is IMO no different from e.g. newer versions of the Java programming language evolving, while still being Java. Other people on Wikipedia disagrees (see Wikipedia talk page there for their side of the argument).
Feel free to also add answers directly to the Wikipedia talk page.