"Substantive", used as a noun, in grammar (or in old grammar terminology) means "noun" or a noun phrase functioning as such in the sentence.

The term "substantif" is still used in French grammar. If you can read French, you can have a look at the following link: http://en.wikipedia.org/wiki/Substantive , in which there is a brief definition of the term "substantif".

Its use as an adjective in English, however, is less common and tends to be found in (as you mentioned) "political speeches or academic contexts". In old grammar terminology, "substantive" was used interchangeably as both a noun and adjective: pertaining to substantives, or a substantive adjective i.e. functioning as a noun in a phrase. In addition, there were "substantive verbs" that expressed existence such as "to be". All this explains the "formality" surrounding the uncommon use of the term as an adjective.