noun Expression in words, either speech or writing.
noun A conversation.
noun A formal lengthy exposition of some subject, either spoken or written.
The preacher gave us a long discourse on duty.
noun Any rational expression, reason.
noun An institutionalized way of thinking, a social boundary defining what can be said about a specific topic (after Michel Foucault).
noun Lengthy, often heated debate over controversial subject matter, particularly within fandom and activist spaces. Sometimes rendered as a proper noun with the definite article (i.e. "the Discourse").